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20. STABLE ISOTOPE AND TRACE ELEMENT GEOCHEMISTRY OF CARBONATE SEDIMENTSACROSS THE CRETACEOUS/TERTIARY BOUNDARY AT DEEP SEA DRILLING PROJECT

HOLE 577, LEG 861

James C. Zachos and Michael A. Arthur, University of Rhode IslandRobert C. Thunell, Douglas F. Williams, and Eric J. Tappa, University of South Carolina2

ABSTRACT

Detailed analyses of well-preserved carbonate samples from across the Cretaceous/Tertiary boundary in Hole 577have revealed a significant decline in the δ1 3C values of calcareous nannoplankton from the Maestrichtian to the DanianAge accompanied by a substantial reduction in carbonate accumulation rates. Benthic foraminifers, however, do not ex-hibit a shift in carbon composition similar to that recorded by the calcareous nannoplankton, but actually increaseslightly over the same time interval. These results are similar to the earlier findings at two North Pacific Deep Sea Drill-ing Project locations, Sites 47.2 and 465, and are considered to represent a dramatic decrease in oceanic phytoplanktonproduction associated with the catastrophic Cretaceous/Tertiary boundary extinctions. In addition, the change in car-bon composition of calcareous nannoplankton across the Cretaceous/Tertiary boundary at Hole 577 is accompanied byonly minor changes in the oxygen isotope trends of both calcareous nannoplankton and benthic foraminifers, suggest-ing that temperature variations in the North Pacific from the late Maestrichtian to the early Danian Age were insignificant.

INTRODUCTION

The uncertain origin of the biotic extinctions at theCretaceous/Tertiary boundary is one of the more intrig-uing problems in paleoceanography. A plethora of mod-els have been proposed to account for the pattern andtiming of the extinctions (e.g., Silver and Schultz, 1982),but many of the published papers are based on relativelyfew data. A recently popular explanation is that of anextraterrestrial cause for the extinctions. An early paperby Alvarez et al. (1980) reported the discovery of anom-alous concentrations of iridium and other platinum groupmetals (siderophile elements) in a clay layer at the Creta-ceous/Tertiary boundary at Gubbio, Italy. Since that time,they and a number of other workers have recorded simi-lar enriched zones at the boundary in at least 19 otherglobally distributed sites, including several terrestrial (non-marine) sequences (Alvarez et al., 1982). It appears that,on the basis of the distribution of the Ir anomalies andthe ratios of platinum group metals to one another, therewas an impact of an extraterrestrial object with the earthat the end of the Maestrichtian Age. However, the sig-nificance of the iridium anomaly as evidence of an im-pact signature has been questioned (e.g., Officer andDrake, 1983; McLean, 1982). Abrupt extinctions of manytaxa of Cretaceous marine plankton occur in conjunc-tion with the Ir anomaly, but the relationship that themajor impact (or other) event had to the biotic extinc-tions is not yet clear.

Alvarez et al. (1980, 1982) proposed a "lights out"scenario, whereby the dust or ejecta thrown up into theatmosphere by the impact would lead to a period of dark-ness lasting several years. This episode of darkness is pro-

Heath, G. R., Burckle, L. H., et al., Init. Repts. DSDP, 86: Washington (U.S. Govt.Printing Office).

2 Addresses: (Zachos, Arthur) Graduate School of Oceanography, University of RhodeIsland, Kingston, RI 02882; (Thunell, Williams, Tappa) Department of Geology, Universityof South Carolina, Columbia, SC 29208.

posed to have led to cessation of photosynthesis, extinc-tion of most phytoplankton, and resulting effects on downthe marine and terrestrial food chains. More recent mod-eling suggests that the period of darkness would havebeen much shorter, on the order of 3 to 6 months (Toonet al., 1982; Thierstein, 1982). Other authors have pro-posed variants of the impact hypothesis, including ex-tinction by chemical poisoning related to the impact of acometary body (Hsü, 1980), catastrophic warming relat-ed to drastic increases in atmospheric water vapor as theresult of an ocean impact and resulting thermal stress onorganisms (Emiliani et al., 1981), and sudden cooling asthe result of global albedo increases from high concen-trations of atmospheric dust (Toon et al., 1982). Thesehypotheses are not mutually exclusive and the suggestedmechanisms could have, in combination, affected the ter-restrial and marine biotas.

Other workers (e.g., Bramlette, 1965; Tappan, 1968;Fischer and Arthur, 1977; Vogt, 1972; McLean, 1980,1982) have provided models for the biotic extinctions atthe Cretaceous/Tertiary boundary that require causes in-ternal to the earth. Vogt (1972), McLean (1980, 1982),and Zoller et al. (1983) suggested increased rates of vol-canism at the end of the Cretaceous. The biotic extinc-tions would therefore be related to either trace metal poi-soning, increased atmospheric pCO2 and sudden globalclimatic warming, and/or decreased oceanic surface wa-ter pH. Others (Bramlette 1965; Tappan, 1968; Fischerand Arthur, 1977) suggested that increased extinction ratesat the end of the Cretaceous were the result of marineregression, oceanic nutrient deficiencies, and global cool-ing. Needless to say, the variety of mechanisms proposedand the often contradictory hypotheses (e.g., warmingvs. cooling) point to the need for much more data bear-ing on rates of extinction, environmental tolerances, andpaleoclimate and paleoproductivity estimates. Thus, highresolution studies of complete marine Cretaceous/Ter-tiary boundary sequences are needed.

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We have examined geochemical and stable isotopiccompositions of sediment and calcareous microfossils inpelagic-hemipelagic sequences across the Cretaceous/Tertiary boundary at a number of sites in the Pacific,the North Atlantic, and the South Atlantic Ocean basins(Zachos et al., unpubl. data; Williams et al., 1983). Ourintent is to provide high resolution chemical and stable-isotopic profiles across the Cretaceous/Tertiary bound-ary in order to evaluate possible changes in carbonatepreservation, which might affect the stable-isotope sig-nals from calcareous plankton, and to evaluate climatic,paleoceanographic or oceanic chemical changes thatmay have occurred at the boundary.

In this chapter we present stable isotopic and geo-chemical data from Cretaceous/Tertiary boundary se-quences recovered at two Deep Sea Drilling Project(DSDP) sites in the Pacific, Sites 47.2 and 577 (Fig. 1).The Site 47.2 sequence is apparently disturbed by drill-ing. Site 577, which was hydraulically piston cored dur-

ing Leg 86 (Site 577 chapter, this volume), offers an un-disturbed Cretaceous/Tertiary boundary transition in apelagic carbonate facies deposited at relatively high sed-imentation rates. Preservation of calcareous microfos-sils is good as we show in scanning electron micrographsand trace element geochemical data (Sr/Ca, Mn) pre-sented in this chapter. We compare our stable-isotoperesults with those from Boersma and Shackleton (1981)from Pacific Site 465 (Figs. 4 and 5). We also discuss theimplications of stable-isotopic data from carbonate fine-fraction, planktonic and benthic foraminifers and changesin carbonate accumulation rates across the boundary forpaleoceanographic events at the end of the Cretaceousthrough the early Paleocene.

ANALYTICAL METHODS

Bulk samples were weighed, disaggregated, and wet sieved througha 63-µm screen. The less than 63-µm fraction was then thoroughlywashed with deionized water through 0.45-µm metricel filters in a vac-uum apparatus. The washed specimens were then prepared for stable-

180c

Figure 1. The paleopositions of Sites 577, 465, and 47.2 in the Northern Hemisphere 70 m.y. ago (after Firstbrook etal., 1979; Lancelot and Larson, 1975). Sites 577 and 47.2 are located on the Shatsky Rise. Site 465 is located on thenearby Hess Rise.

514

GEOCHEMISTRY OF CARBONATE SEDIMENTS

isotope analysis in the following manner. The samples were first roast-ed at 400°C under vacuum for 1 hr. and then reacted in 100% ortho-phosphoric acid at 50°C. Isotopic analyses were preformed on a VGMicromass 602D isotope ratio mass spectrometer. All samples weremeasured relative to a laboratory standard reference gas and are re-ported in notation as per mil deviations from PDB.

δ(‰) =R standard

- 1 \ × I03

Calibration of the reference gas was achieved by analysis of B-l stan-dard, which has a measured value of + 0.06‰ for δ 1 8 θ and + 0.64‰for δ1 3C relative to PDB (Keigwin, 1980). Precision of isotopic analy-ses is better than + 0.11 per mil for oxygen and + 0.08 per mil for carbon.

Trace-element measurements were conducted on the less than 63µm carbonate fraction. Each sample was thoroughly washed with de-ionized water to remove sea salts, dried, and finely ground. The sam-ples (0.1 g) were then dissolved in 0.7 N acetic acid to avoid strippingof ions from clay minerals as much as possible, filtered, and diluted to100 ml. Magnesium, strontium, and manganese concentrations werethen determined with a Perkins Elmer Model 503 Atomic AbsorbtionSpectrophotometer. All results are reported in ppm. Analytical preci-sion for Mg is ± 5 % , for Mn ± 3 % , and for Sr ± 8 % . Although we al-so analyzed the same samples for Ca, the analytical precision was muchlower (±20%) than for the other elements. We have used calculatedCa values from carbonate contents, assuming pure CaCO3. This tech-nique has worked well for other Cretaceous pelagic carbonate sequenceswhere analytical Ca and total CaCO3 contents were obtained (Deanand Arthur, unpubl. data) and introduces less variation due to poorprecision than the atomic absorption method. Percent calcium car-bonate was determined by the bomb method (Müller and Gastner,1971).

Carbonate accumulation rates (CAR) (Table 4) were determinedusing the following equation:

CAR = (%CaCO3/100){[(Wg/cm3)1 g/cm3)](5 cm/yr.)}

where P is porosity and W is wet bulk density. Sedimentation rates (S)were determined from magneto- and biostratigraphies.

STABLE-ISOTOPE RESULTS

Planktonic Calcareous Microfossils

Results of oxygen- and carbon-isotope analyses of Mae-strichtian and Danian fine fraction (<63 µm) CaCO3

and benthic foraminifers from Hole 5773 and whole rockCaCO3 from Site 47.2 are given in Tables 1 and 2 andshown in Figures 2 and 3. Carbon- and oxygen-isotopeanalyses were performed on samples of carbonate finefraction from the top of Section 577-12-2 {Chiasmolith-us danicus, nannofossil Zone CP2, 101-106 m sub-bot-tom depth) to the bottom of Section 577-13-4 {Miculamums nannofossil Zone, 109-117.6 m sub-bottom depth)at Hole 577 and from the top of Section 47.2-10-6 {Cru-ciplacolithus tenuis, CPlb Subzone) to the bottom ofSection 47.2-12-2 (M. murus Zone) at Site 47.2. Samplespacing ranged from 10- to 20-cm intervals although thespacing interval was reduced to 5 cm near the Cretaceous/Tertiary boundary at each site.

Oxygen-isotope values of Hole 577 carbonate fine-fraction samples from the M. murus zone varied only

Subsequent to completion of this manuscript a recalibration of standard values wasconducted at the Graduate School of Oceanography stable-isotope lab which affects the con-version of measured values to PDB. As a result, Hole 577 oxygen and carbon stable-isotopevalues reported here in tables, text, and illustrations should be corrected by adding 0.42‰ and0.13‰ respectively. The change in oxygen-isotope values amounts to less than 2°C in paleo-temperature estimates.

slightly upcore, averaging - 1 . 0 ± O.O5%o throughout(Fig. 2). Values begin to increase slightly near the top ofthis zone several centimeters below the Cretaceous/Ter-tiary boundary, reaching a peak of - O.49%o at Sample577-12-5, 99 cm. Above this level, through the Markali-us inversus and C. tenuis zones (106 to 109 m sub-bot-tom depth), δ 1 8 θ values vary considerably, fluctuatingbetween a low of - 1.45%o in Sample 577-12-3, 110 cmand high value of - 0.16%o in Sample 577-12-4, 110 cm.

Site 47.2 Maestrichtian δ 1 8 θ values are only an aver-age of O.l%o heavier than the δ 1 8 θ values of correspond-ing samples from Hole 577 (Figs. 3 and 4). Similarly, theDanian oxygen-isotope ratios are also just slightly heavierthan the corresponding values from Hole 577.

δ1 3C values of carbonate fine-fraction samples fromthe M. murus Zone are relatively constant, averaging2.65 ± O.2%o (Fig. 2). At the Cretaceous/Tertiary bound-ary, there is a sharp decrease of l.O‰ over a 10-cm in-terval which coincides with a layer of slightly lower thanaverage carbonate content. Further upcore, through theC. tenuis nannofossil Zone, δ13C values remain relativelylow between 1.7 and 2.3%o A similar shift in δ1 3C is re-corded in the Site 47.2 bulk CaCO3 samples (Fig. 3).Maestrichtian samples at Site 47.2 yield average δ13C val-ues of 2.6 ±O.2%o compared to values of 1.75 ± 0.1%0

for Danian samples, with the actual carbon-isotope shiftoccurring somewhere between Samples 47.2-11-5, 25 cmand 47.2-11-3, 100 cm. The major shift apparently coin-cides with the Cretaceous/Tertiary boundary determinedfrom calcareous nannofossils (Thierstein, 1981). Anom-alous points in both the Maestrichtian and Danian sec-tions are probably an artifact of displaced or mixed sed-iments resulting from coring disturbances.

Stable carbon-isotope compositions of planktonic for-aminifers from Site 465 (Boersma and Shackleton, 1981)on the Hess Rise have been plotted against an absolutetime framework in Figure 5 along with the data fromSites 577 and 47.2 for comparison using the chronologyshown in Figure 6. Boersma and Shackleton (1981) de-termined Maestrichtian and Danian near-surface-watercarbon-isotope values at Site 465 by analyzing Rugoglo-bigerina and Guembelitria, respectively. The absolute shiftin δ1 3C (surface water) at Site 465 as recorded in the testsof these planktonic foraminifers is greater (2.2%o) thanthe shifts found at Sites 577 and 47.2 as measured in thefine fraction CaCO3 (Fig. 5). However, because the car-bon-isotope anomalies at Hole 465 were not determinedon a single species across the boundary, this rather largeshift may be partially a function of differences in speciesvital effects, although it is unlikely that the entire shiftcould be attributed to this. A further possibility is thatthe measured values reflect actual differences in the car-bon composition of surface waters overlying these twolocations; however, given the relative proximity of theShatsky and Hess rises and their central paleoposition inthe Pacific, this also seems unlikely.

Benthic Foraminiferal Isotope Records

Since many species of benthic foraminifers survivedthe Cretaceous/Tertiary boundary extinctions (Douglasand Woodruff, 1981), one may assume that it would be

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J. C. ZACHOS ET AL.

Table 1. Stable-isotope results from Hole 577.

Core-Section(interval in cm)

12-2, 49-5112-2, 69-7112-2, 89-9112-2 99-10112-2, 129-13112-3, 9-1112-3, 29-3112-3, 49-5112-3, 69-7112-3, 89-9112-3, 109-11112-3, 129-13112-4, 9-1112-4, 29-3112-4, 49-5112-4, 69-7112-4, 89-9112-4, 109-11112-5, 19-2112-5, 39-4112-5, 59-6112-5, 79-8112-5, 91-9312-5, 98-10012-5, 108-10912-5, 113-11412-5, 117-11912-5, 125-12612-5, 128-13012-5, 131-13212-5, 134-13612-5, 144-14512-6, 19-2112-6, 39-4112-6, 59-6112-6, 99-10112-7,9-1113-1, 59-6113-1, 99-10113-1, 139-14113-2, 39-4113-2, 79-8113-2, 119-12113-3, 19-2113-3, 59-6113-3, 99-10113-3, 139-14113-4, 39-4113-4, 99-10112-2, 89-9112-2, 129-13112-3,9-1112-4, 89-9112-5, 59-6112-5, 79-8112-5, 117-11912-5, 125-12612-5, 108-10912-5, 128-13012-6, 19-2112-6, 59-61

Depth(m)

103.8104104.2104.3104.6104.9105.1105.3105.5105.7105.9106.1106.4106.6106.8107107.2107.4108108.2108.4108.6108.72108.89108.99109.44109.09109.16109.19109.22109.26109.35109.6109.8110110.4111111.8112.2112.6113.1113.5113.9114.4114.8115.2115.6116.1116.7

•% CaCθ3

9391.991.692.894.589.793.991.392.587.994.8592.795.495.693.195.694.595.293.191.393.193.79391.593.293.589.588.288.894.895.79695.296.194.895.293.687.49495.3896.394.594.89494.690.995.194.194.9

% > 63 µtn

71133311113

1114182027252320282524141615139631

< l< l< l< l< l< l< l< l< l< l< l< l< l< l< l< l< l< l< l

Sample type

Fine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fractionFine fraction

AragoniaAragoniaAragoniaAragoniaAragoniaAragoniaAragoniaAragoniaAragoniaAragoniaAragoniaAragonia

δ 1 8 O P D B

-1.30-1.42-1.37-0.90-1.27-1.24-0.97-1.08-0.95-1.37-1.45-1.30-1.09-1.13-1.24-0.85-0.34-0.16-0.85-0.95-0.90-0.87-0.70-0.49-0.83-0.63-0.65-0.73-0.74-0.82-0.64-0.97-0.81-0.85-0.87-0.93-0.94-1.08-1.05-0.99-1.10-1.03-1.05-0.97-0.94-0.95-0.93-0.99-0.87-0.05

0.08-0.08

0.050.480.390.200.150.450.040.31

-0.33

6 1 3 C P D B

2.031.952.132.092.021.892.042.182.272.061.922.042.001.981.731.751.871.851.781.831.841.982.192.152.022.021.912.032.492.832.942.872.762.552.552.502.642.482.492.832.802.782.762.722.802.652.862.712.640.810.880.60

.21

.98

.90

.65

.61

.82

.651.360.82

fairly easy to reconstruct monospecific benthic stable-isotope records across the boundary. However, speciesof benthic foraminifers that are abundant in Danian sam-ples are often present but not abundant in Maestrichtiansamples and vice versa. In addition, the dilution of coarse-fraction foraminifers by calcareous nannofossils (pre-dominance of fine fraction) in Maestrichtian sedimentsmakes it extremely difficult to find the necessary num-ber of a specific benthic foraminifer from the small sam-ple sizes available at the closely spaced intervals neededto conduct stable-isotope analyses. In this case, analyses

of mixed benthic assemblages, which provide less reli-able data, must be performed instead. At Hole 577 wewere fortunate enough to find one genus of benthic for-aminifer, Aragonia (Plate 2, Figs. 1, 3, and 4), that wasboth relatively plentiful and present in a few samplesfrom either side of the boundary. The measured δ1 3Cvalues of late Maestrichtian Aragonia indicate a carbon-isotope trend toward heavier values just below the bound-ary, increasing from +O.83%o a meter below the bound-ary (Sample 577-12-6, 79 cm) to -I- 1.38%o at the bound-ary (Sample 577-12-5, 125 cm). This trend continues in

516


Table 2. Stable-isotope results from Site 47.2.


10-6, 50-5110-6, 100-10110-6, 127-12811-1, 25-2611-1, 75-7711-1, 125-12611-2, 30-3111-2, 50-5111-2, 100-10111-3, 30-3111-3,60-6111-3, 100-10111-3, 125-12611-3, 138-13911-4, 30-3111-4, 60-6111-4, 80-8111-4, 98-9911-4, 120-12111-4, 138-13911-5, 10-1111-5,20-2111-5, 27-2811-5, 30-3111-5, 48-4911-5, 70-7111-5, 90-9111-5, 110-11111-5, 127-12811-5, 140-14111-6, 23-2411-6,40-4111-6, 60-6111-6, 80-8111-6, 125-12612-1, 25-2612-1, 75-7612-1, 125-12612-2, 25-2612-2, 68-6912-2, 100-10112-2, 125-127

"la > 63 µm

484

161920182018

17

151614

143

104955

< l< l< l< l< l< l< l< l< l< l< l< l< l< l< l< l< l< l< l< l

Sample type

Whole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rockWhole rock

δ 1 8 o P D B δ 1 3

-0.97 :- i . i 3 ;-1.31-1.15-0.86-0.98-0.72-0.76-0.61-0.74 :-0.74 :-0.76-0.70-0.67-0.97 :-0.92 :-0.88 :-0.83 :-0.85 ;-o.9i :-0.94 :-o.9i :-0.90 ;-o.9o :- I . O I :-0.78 :-0.85 :-0.87-0.92-0.77-0.89 :-0.78 :-0.84 :-0.70-0.89-0.92-1.10-0.98 :-0.93-1.01-1.02-1.07

CPDB

>.051.04.99.38.39.62.66.71.93

'.802.19.96.90.90

'.662.662.642.102.141.28'.482.522.432.762.42

2.69

5.731.752.65

2.65

2.58

5.57

2.742.72

1.74

5.522.39

2.79

2.81

2.73

2.752.75

the Danian section, reaching a maximum value of 1.98%oin Sample 577-12-5, 60 cm. Further upcore, in Sections577-12-2 and 577-12-3, Aragonia values return to pre-boundary levels, decreasing by as much as 1.2‰.

A similar trend across the Cretaceous/Tertiary bound-ary, but of lesser magnitude, is exhibited in the Site 465carbon-isotope values of another benthic foraminifer, Bu-limina (Fig. 4; Boersma and Shackleton, 1981). The δ13Cvalues of Maestrichtian Bulimina become enriched byO.5%o, relative to earlier late Maestrichtian values, priorto the boundary. Above this point, the carbon isotopiccomposition of Bulimina decreases by about O.4%o up to65.5 m.y. ago (above the Cretaceous/Tertiary boundary).There appears to be a slight discrepancy between thetwo benthic isotope records in both the overall trend andtime of change, although the overall magnitude of theisotope values are similar. Because the analyses of ben-thic foraminifers at these two sites were performed ondifferent species, Bulimina and Aragonia, the discrep-ancies between the two isotope records may be attribut-able to differences in species vital effects. In fact, differ-ent species of modern benthic foraminifers from the samesamples have been found to possess significantly differ-

ent δ1 3C values and may also show considerable intra-specific variation between various size fractions (Wood-ruff et al., 1980; Belanger et al., 1981; Savin and Yeh,1981).

Oxygen-isotope analyses of Aragonia in Hole 577 showno prominent trends (range ~O.5%o) although addition-al analyses of Core 13 samples should be conducted inorder to determine whether the anomalous Maestrichti-an value (Sample 577-12-6, 70 cm) is significant (Fig. 2).Overall, the δ 1 8 θ values are 0.8-1.5%o heavier than thevalues obtained for planktonic organisms and roughlyequivalent to the values obtained for Site 465 Bulimina(Fig. 4).

TRACE-ELEMENT GEOCHEMISTRY

Mineralogic stabilization of carbonate sediment oc-curs through solution-recrystallization processes. Theseprocesses also result in carbon- and oxygen-isotope sta-bilization. The stable-isotope values of altered carbon-ate sediments, therefore, may not represent the isotopiccomposition of the originally precipitated calcite, butrather some average of the isotopic composition of theoriginal calcite and the reprecipitated calcite cements andovergrowths. In order to determine the extent of recrys-tallization, early investigators generally examined thedownhole changes of various textural parameters suchas porosity and cementation. However, more recently,chemical criteria have been considered in conjunctionwith textural studies, providing investigators with a quan-titative as well as a qualitative measure of diagenesis (e.g.,Matter et al., 1975). The distribution and concentrationof certain trace elements such as strontium, magnesium,and manganese are known to change with increasinggrades of diagenesis (e.g., Veizer, 1978; Baker et al., 1982).The direction of change is generally dependent upon thedistribution coefficient of the element in question. Al-though contamination by noncarbonate fractions mayprevent precise measurement of some elements such asMn, the overall downhole trends and their signs provideus with important information for evaluating oxygen-isotope data that may have been influenced by diagene-sis. The concentrations of the trace elements strontium,magnesium, and manganese in fine-fraction carbonatesfrom across the Cretaceous/Tertiary boundary at Hole577 are therefore reported and evaluated here (Table 3,Fig. 7).

The average measured value of magnesium for Site577 samples falls within the typical range of values ex-pected for a nannofossil ooze (Bathurst, 1981). From116 to 109 m (the Cretaceous/Tertiary boundary) sub-bottom depth Mg concentrations range from 800 to1300 ppm, varying very little throughout. From 109 to108 m sub-bottom depth Mg concentrations increasesharply, reaching a peak value of 2100 ppm in Sample577-12-5, 20 cm. This increase in Mg concentration co-incides with an increase in fine-fraction insoluble residuecontents (Fig. 7) over the same interval, suggesting thatsome Mg ions were leached from the clays during sam-ple preparation. Above the Cretaceous/Tertiary bound-

517

J. C. ZACHOS ET AL.

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CaCO3(%)

2.5 85 90 95 100

A Aragonia (benthic foraminifer)

• Fine fraction CaCO3 K62 µm)

Figure 2. Carbon- and oxygen-isotope values of Hole 577 fine-fraction CaCO3 and benthic foraminifer Aragoniaand the carbonate content of whole rock. The Cretaceous/Tertiary boundary is located at Sample 577-12-5, 130cm (see Monechi, this volume). OG = organic geochemistry sample, CC = core catcher.

ary (108 to 105 m) Mg concentrations decline to valuesof 1100 ppm, similar to those for the Maestrichtian sam-ples.

Manganese concentrations are generally low through-out the entire sequence, ranging between 50 and 200 ppm(Fig. 7). Higher concentrations in Danian samples cor-respond to higher levels of insoluble residue, again sug-gesting that the measured concentrations of Mn wereprobably inflated by additional leached ions from man-ganese micronodules or other particles associated withthe insoluble residue fraction.

Strontium content (and Sr/Ca ratio) varies very littlein samples from below the boundary, ranging from 950(Sample 577-12-6, 100 cm) to 1230 ppm (Sample 577-13-4, 100 cm) (Fig. 7). A sharp increase in Sr occurs justabove the boundary between Samples 577-12-5, 109 cmand 577-12-5, 92 cm, with concentrations reaching 1700ppm. Sr content declines rapidly upcore and, unlike Mgor Mn, exhibits no correspondence with the insoluble

residue concentration. The Sr values and the Sr/Ca ra-tios are those expected for moderately preserved pelagiccarbonates (Matter et al., 1975; Scholle, 1977).

CARBONATE PRESERVATION

In order to evaluate further the degree of preservationof Site 577 carbonates, scanning electron microscope(SEM) photographs were taken of foraminifer and coc-colith tests and ultrastructure in conjunction with trace-element studies.

Adelseck et al. (1973) have demonstrated in labora-tory experiments that calcite overgrowths and dissolu-tion etching occur with greater frequency with increas-ing burial conditions (i.e., increased temperature and pres-sure). The dissolution and recrystallization of calcite ina calcareous ooze is such that cement overgrowths onthe low magnesium calcite of large coccoliths and disco-asters occur at the expense of planktonic foraminifersand small coccoliths, which usually disaggregate with

518


100-

1α105-

εo0

Sub

-b

110-i

<

BC

0

Pali

cra

itri

cht

i

o( J

10

11

12

V)

6 "

1

2

3

4

5

6

1

2

CaCO, (%)

100

Planktonicforaminiferal

zonationCoccolithzonation

Subbotinatrinidadensis

——' Eoglobigerina '

Aplanktonic zone

Globigerina eugubina

Globotruncanamayaroensis

Cruciplacolithustenuis

Tetralithusmurus

Figure 3. Site 47.2 carbon- and oxygen-isotope values and carbonate content of whole rock samples. Included are Cretaceous/Tertiary boundarylocations based on planktonic foraminifer (dots; Hofker, 1978) and coccolith (dashed; Thierstein, 1981) biostratigraphy.

2.0 -1.5

δ 1 8 O p D B ( % o )

-1.0 -0.5 0 0.5 1.0

65.0-

65.5-

66.0-

66.5-

67.0

• Site 577 fine fraction π Site 577 Aragonia (benthic foraminifer)

A Site 465 planktonic foraminifers O Site 465 Bulimina (benthic foraminifer)

• Site 47.2 whole rock (Boersma and Shackleton, 1981)

Figure 4. Cretaceous/Tertiary planktonic and benthic oxygen-isotope records from three NorthPacific DSDP sites: 465 (data from Boersma and Shackleton, 1981), 47.2, and 577. Site465 is located on the Hess Rise and 47.2 and 577 on the Shatsky Rise (see Fig. 1).

519

J. C. ZACHOS ET AL.

65.0-

67.0

• Site 577 fine fraction

• Site 47.2 whole rock

* Site 465 planktonic foraminifers(Boersma and Shackleton, 1981)

G Site 577 Aragonia

O Site 465 Bulimina(Boersma and Shackleton, 1981)

Figure 5. Compilation of planktonic and benthic carbon-isotope records from Sites 465 (data fromBoersma and Shackleton, 1981), 47.2, and 577.

increasing dissolution (e.g., Douglas and Savin, 1971;Schlanger and Douglas, 1974; Roth and Berger, 1975;Berger, 1975; Thierstein, 1980; Thunell, 1982). As dia-genesis progresses, cementation of grains generally fol-lows, resulting in a transition from ooze to chalk to lime-stone. A well-preserved calcareous fossil assemblageshould, therefore, exhibit the following physical charac-teristics: (1) individual specimens of foraminifers andnannofossils are essentially unbroken and free of exten-sive etchings or overgrowths, and (2) cementation be-tween grains is absent. If signs of extensive diageneticalteration do exist, then stable-isotope values must beconsidered suspect.

SEM photographs of Site 577 Danian and Maestrich-tian samples show moderately to well-preserved calcare-ous microfossil specimens. Coccoliths from Maestrichti-an samples in Hole 577 (Plate 4, Figs. 5-8) exhibit somesigns of incipient dissolution and recrystallization.

Overall, the preservation of Maestrichtian calcareousnannofossils is moderate at Site 577. Euhedral calciteovergrowths are present on some coccoliths and somespecimens are etched or partially dissolved and disag-gregated (Plate 4, Figs. 7 and 8). Similar structures havebeen found on coccoliths from Site 47.2 (Plate 1, Fig. 3and 4). Earlier calcareous nannofloral work at Site 47.2(Thierstein, 1981) revealed a large increase in the pro-portions of dissolution-resistant taxa, such as Micula stau-rophora and M. murus in the middle sections of Core 11,suggesting either slight pre- or postburial dissolution.However, because the sediments at Site 47.2 have been

mechanically displaced and mixed, it is difficult to de-termine exactly where stratigraphically, relative to the Cre-taceous/Tertiary boundary, the dissolution may have oc-curred.

Danian samples from both Sites 577 and 47.2 (Plate1, Fig. 2; Plate 2, Figs. 1-4) also contain moderately towell-preserved specimens of calcareous nannofossils. Over-growths are notably absent on many coccoliths from Sam-ple 577-12-2, 50 cm (Plate 3), although some coccolithsat that level are overgrown and/or etched, especially onthe inner regions of the proximal side of coccolith shields(Plate 3, Fig. 3). Calcareous nannofossils at the Creta-ceous/Tertiary boundary appear to be just as well, if notbetter, preserved than those from above and below theboundary (Plate 4, Figs. 1-4), as was observed at Site384 by Thierstein and Okada (1979). In addition, unlikeAragonia from other sections of the core (Plate 2, Fig.5), the tests of Aragonia from the Cretaceous/Tertiaryboundary samples show fewer adhering cemented car-bonate fragments and/or coccoliths (Plate 2, Fig. 6). Fur-ther examination of the carbonate trace-element data re-veals that Sr/Ca ratios increase over a short intervalacross the Cretaceous/Tertiary boundary (Fig. 7). Thisprovides more substantial empirical evidence on the stateof preservation of the fine-fraction carbonates. Duringdissolution-reprecipitation of calcite, Sr ions are prefer-entially removed from the carbonate matrix (because theSr partition coefficient is less than 1) so that with pro-gressive recrystallization, cementation, or replacement theSr/Ca ratio of the calcite should decrease (Lawrence,

520


65-

66-

Cret./Tert.

68H

AgeM

ae

stri

ch

tian

I D

an

ian

Nannoplanktonzones

Chiasmolithusdanicus(NP3)

Cruciplacolithustenuis(NP2)

Markaliusinversus

(NP1)

Miculaprinsii

Micula murus

Lithraphiditesquadratus

Planktonicforaminifer

zones

Morozovellauncinata

(P2)

Subbotinatrinidadensis

(P1c)

Subbotinapseudobulloides

(Pib)

Globigerina~~\ eugubina f~

\ (P1a) /

mayaroensis

_/ Globotruncana \ Jgansseri

Magneticstratigraphy

Polaritychrons

•

••

!

27

28

29

30

31

GubbioL+

K-

J +

I-

H +

G-

F+

F 2 ~

F 3

Table 3. Site 577 trace-element concentrations (<63 µmcarbonate).

Figure 6. The Cretaceous/Tertiary boundary time scale and zonationsutilized in this investigation. After van Hinte (1976), Martini (1976),Palmer (1983), Poore et al. (1983), and Berggren, Kent, and Flynn(in press). Polarity chrons are defined in Alvarez et al. (1977).

Core-Section

(interval in cm)

12-2,49-51

12-2, 129-13112-3, 49-51

12-2, 129-13112-4, 49-51

12-4, 89-9112-5, 19-2112-5, 59-6112-5, 91-9312-5, 108-109

12-5, 125-12612-5, 128-13012-5, 131-13212-5, 144-145

12-6, 39-41

12-6, 99-10113-1, 59-6113-1, 139-141

13-2, 79-81

13-3, 59-6113-3, 139-141

13-4, 99-101

«/o CaCO 3

(bulk)

90.3

90.587.4

92.890.587.184.3

87.194.294.185.488.495.1

95.895.8

96.5959596.494.293.7

93

Sr

(ppm)

1050

10301190

10001030

1010126012601640

16701030113010801130

1010940

1190UIO

115011401210

1230

Mg(ppm)

1010950

1220

12601610

18202080200018401760159012001010

890960

11801380168013101390

15701500

Mn(ppm)

145

168201139112

148

149149126118150

10115912254

5379534954

68

113

1971; Matter et al., 1975; Sayles and Manheim, 1975;Scholle, 1977; Brand and Veizer, 1980; Bathurst, 1981;Baker et al., 1982). However, it is difficult to imaginewhy the Cretaceous/Tertiary boundary carbonate wouldbe better preserved than the carbonates in overlying se-quences, considering that sedimentation rates were ap-parently lower, and other workers have suggested a ma-jor dissolution pulse at the boundary (e.g., Worsley, 1974).Some workers suggest that chemical preservation of

105-

106-

107-

108-

109-

De

pt

112-

113-

114-

115-

116-

Cf)

<

c

8£α.

rich

tiar

S

late

M

oO

12

13

Sec

3

4

5

6

1

2

3

4

Mn (ppm)

100 200

Mg (ppm) Sr/Ca x I0"4 CaCO j (%)

1000 2000 20 30 40 50 70 80 90 100

Fine-fractionCaCO3 as apercentageof whole rock

Fine-fractionCaCO j as apercentageof fine fraction

Figure 7. Manganese (Mn) and magnesium (Mg) concentrations and strontium/calcium ratios (Sr/Ca) of thefine-fraction (<63 µm) carbonates from Cores 12 and 13 of Hole 577. Fine-fraction carbonate content ofwhole fine-fraction and whole rock are included. The boundary is located at 109 m sub-bottom depth.

521

J. C. ZACHOS ET AL.

CaCO3 may be enhanced if the lithology of the sedimentsequence is not conducive to cementation (i.e., low po-rosity, high clay content, etc., Schlanger and Douglas,1974; Matter et al., 1975). Downhole variations in pres-ervation, therefore, may often be a function of changesin insoluble residue content. The increase in insolubleresidue at the boundary represents a slight change in li-thology, one which may have improved the resistance ofthe boundary sequence to postburial overgrowth and ce-mentation. This is in contrast to reports of poorly pre-served calcareous microfossils at some other Cretaceous/Tertiary boundary sequences (Worsley, 1974; Monechi,1977).

An important consideration is whether or not there isany relationship between the changes in the state of pre-servation and the anomalies in the oxygen-isotope rec-ord. The slight enrichment in δ 1 8 θ at the boundary maybe interpreted as improved preservation (no addition ofisotopically lighter cement) rather than as a decrease inthe temperature of the surface water mass in which thecalcite was precipitated. Downhole changes in Sr/Ca ra-tios and preservation have been correlated previously withchanges in oxygen-isotope records (Matter et al., 1975),whereas others have developed simple mathematical mo-dels attributing decreasing trends in δ 1 8 θ records throughtime to progressively greater diagenetic reactions withincreasing age and burial depth (Killingley, 1983). Ignor-ing temperature effects for the moment, the somewhatanomalous δ 1 8 θ values of the Hole 577 boundary sam-ples may be a function of the degree of diagenesis. If so,similar enrichments in oxygen-isotope signals at othermarine boundary sequences (Site 524, Hsü et al., 1982a;Site 516, Williams et al., 1984) may also be a function ofdifferential preservation (due to high clay content) ratherthan the result of real changes in oceanic temperatures.

CARBONATE ACCUMULATION RATES

Barring severe diagenesis and/or preburial dissolution,carbonate accumulation rates on the seafloor should re-flect the magnitude of productivity in overlying surfacewaters. One would therefore expect that a postulated de-crease in phytoplankton production associated with theCretaceous/Tertiary boundary extinctions would be ac-companied by a similar decrease in carbonate accumula-tion rates. At Hole 577 total and fine-fraction carbonateaccumulation and rates decline significantly across theCretaceous/Tertiary boundary (Table 4) (Fig. 8). Pre-boundary rates are stable at 1.10 g/cm2/1000 yr., withfine-fraction carbonate accumulation contributing a ma-jor portion to the total (>99%). Just above the bound-ary, total carbonate accumulation rates decrease to 0.3g/cm2/1000 yr., with fine-fraction carbonate contribut-ing far less as a percentage of the total (75-85%). Thismay imply that the decline in fine-fraction carbonate ac-cumulation was more severe relative to the decline in for-aminifer accumulation, that the recovery of foraminiferproduction occurred earlier than that of nannofossil (phy-toplankton) production, or that current winnowing offine-fraction carbonate occurred relative to coarse frac-tion.

There do not appear to be any significant variationsin carbonate accumulation rates during the early Paleo-cene because values generally remain constant. However,accumulation rates for the entire Cruciplacolithus tenuisand Markalius inversus zones (Samples 577-12-5, 128 cmto 577-12-4, 70 cm) were determined by utilizing a singlesedimentation rate of 2.9 m/m.y. This rate was deter-mined by using the lowest datable biostratigraphic mark-er (first appearance datum Chiasmolithus danicus) avail-able. It is probable, however, that actual sedimentationrates varied considerably over this interval. For example,the 10-20 cm of sediment lying just above the boundarymay have accumulated at a much slower rate than thecalculated rate of 2.9 m/m.y., by inference from the high-er insoluble residue contents and the possibility that sedi-ments higher in the C. tenuis Zone may have accumu-lated at a greater rate. This may explain why calcula-tions reveal an increase in foraminifer accumulation ratesacross the boundary whereas, given the magnitude of thebiotic extinctions, one might expect a decline. However,unless datable biostratigraphic markers lower than theone used to determine the average 2.9 m/m.y. rate arelocated, the aforementioned rate must, for the moment,be considered the best obtainable. Sedimentation ratesas determined with preliminary paleomagnetic stratigra-phy (Fig. 9) (Bleil, this volume) are similar to the aboverates.

We believe that the magnitude of the decrease in thecarbonate accumulation rate is real and not only a func-tion of the absolute time scale used. First of all, the ac-cumulation rate of insoluble residue (Fig. 7) does notdecrease across the Cretaceous/Tertiary boundary by thesame factor as the carbonate accumulation rate. Second,the insoluble residue accumulation rate decreases onlygradually from Maestrichtian through Danian time.

DISCUSSION

As reported at Site 465 (Boersma and Shackleton, 1981)there appears to be little if any temperature change insurface waters across the Cretaceous/Tertiary boundaryat Hole 577. The slight enrichment in the CaCO3 fine-fraction δ 1 8 θ in Hole 577 is perhaps an artifact of pres-ervation-diagenesis rather than a function of surface wa-ter temperature changes. However, if one were to inter-pret this O.5%o enrichment in terms of a temperaturechange, such an enrichment in δ 1 8 θ would represent a2°C decrease in surface water temperatures at most. Asecond enrichment occurs in the lower part of Section577-12-4 and may represent a slightly greater tempera-ture decrease (3-3.5°C). Higher in the core, δ 1 8 θ valuesshow significant fluctuations and are difficult to inter-pret. In Section 577-12-3, δ 1 8 θ values are slightly en-riched, identical to average Maestrichtian values. How-ever, the δ1 3C ratios and the ratios of coarse-fraction(<63 µm) to fine-fraction carbonate over this intervalare also similar to ratios in Maestrichtian sediments, in-dicating a possible lateral reworking of sediments throughsome sections at this location. The presence of poorlypreserved fragments of Maestrichtian foraminifers in Sec-tion 577-12-3 is confirmation that reworking occurred.

522


Table 4. Site 577 accumulation rates.


12-2, 49-5112-2, 69-7112-2, 89-9112-2, 109-11112-2, 129-13112-3, 9-1112-3, 29-3112-3, 49-5112-3, 69-7112-3, 89-9112-3, 109-11112-3, 129-13112-4,9-1112-4, 29-3112-4, 49-5112-4, 69-7112-4, 89-9112-4, 99-10112-5, 19-2112-5, 39-4112-5, 59-6112-5, 79-8112-5, 83-8512-5, 98-10012-5, 108-10912-5, 113-11412-5, 117-11912-5, 125-12612-5, 128-13012-5, 131-13212-5, 134-13612-5, 139-14012-5, 144-14512-6,9-1112-6, 19-2112-6, 29-3112-6, 39-4112-6, 49-5112-6, 59-6112-6, 69-7112-6, 79-8112-6, 89-9112-6, 99-10112-6, 109-11112-6, 119-12112-6, 129-13112-6, 139-14112-7,9-1113-13-13-13-13-13-13-

, 19-21, 39-41, 59-61, 79-81, 99-101, 119-121, 139-141

13-2, 19-2113-2, 39-41,13-2, 59-6113-2, 79-8113-2, 99-10113-2, 119-12113-2, 139-14113-3, 19-2113-3, 39-4113-3, 59-6113-3, 79-8113-3, 99-10113-3, 119-12113-3, 139-14113-4, 19-2113-4, 39-4113-4, 59-6113-4, 79-8113-4, 99-10113-4, 119-12113-4, 139-141

Depth(m)

103.8104104.2104.4104.6104.9105.1105.3105.5105.7105.9106.1106.4106.6106.8107107.2107.3108108.2108.4108.6108.64108.89108.99109.44109.09109.16109.19109.22109.26109.29109.35109.5109.6109.7109.8109.9110110.1110.2110.3110.4110.5110.6110.7110.8111111.4111.6111.8112112.2112.4112.6112.9113.1113.3113.5113.7113.9114.1114.4114.6114.8115115.2115.4115.6115.9116.1116.3116.5116.7116.9117.1

Age(m.y.)

64.5564.6264.6964.7664.8364.936565.0765.1465.2165.2865.3465.4565.5265.5965.6665.7265.766666.0766.1466.2166.2266.3166.3466.5066.3866.466.4066.4166.4166.4166.4266.4366.4466.4566.4666.4766.4866.4966.5066.5166.5266.5366.5466.5566.5666.5866.6266.6466.6666.6866.7066.7266.7466.7766.7966.8166.8366.8566.8766.8966.9266.9466.9666.9867.0067.0267.0467.0767.0967.1167.1367.1567.1767.19

WBD(g/cm3)

1.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.651.65

Poros.

0.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.50.5

CaCO3

W

0.930.9190.9160.9380.9450.8970.9390.9130.9250.8790.94850.9270.9540.9560.9310.9560.9450.990.9310.9130.9310.9370.930.9150.9320.9350.8950.8820.8880.9480.9570.9590.960.9520.9520.9580.9610.9390.9480.950.9690.9640.9520.9440.9450.9240.9440.9360.9460.9430.8740.9510.940.9520.95380.9040.9630.9610.9450.9490.9480.950.940.9530.9460.9740.9090.9640.9510.9240.9410.950.9480.9490.9280.952

DBD(g/cm3)

1.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.151.15

Sed. rate(cm/1000 yr.)

0.290.290.290.290.290.290.290.290.290.290.290.290.290.290.290.290.290.290.290.290.290.290.290.290.290.290.29

llll

MAR

0.330.330.330.330.330.330.330.330.330.330.330.330.330.330.330.330.330.330.330.330.330.330.330.330.330.330.331.11.11.11.11.11.11.11.11.11.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.11.11.11.11.11.11.11.11.1

CAR

0.310.310.300.310.310.300.310.300.310.290.310.310.320.320.310.320.310.330.310.300.310.310.310.300.310.310.301.01.01.11.11.11.11.11.11.11.11.1.1.1.1.1.1.1.1.1.

.

1.11.11.11.01.11.1.1.l.(1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.

)

)

Wt. %<63 µm

0.930.990.990.970.970.970.990.990.990.990.970.890.860.820.80.730.750.770.80.720.750.760.830.840.850.870.910.940.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.990.99

FFAR(g/cm2/1000 yr.)

0.2870.3020.3010.3020.3040.2890.3090.3000.3040.2890.3060.2740.2720.2600.2470.2320.2350.2530.2470.2180.2320.2360.2560.2550.2630.2700.2700.9491.011.071.081.091.091.081.081.091.091.061.071.081.101.091.081.071.071.051.071.061.071.070.9911.081.071.081.081.021.091.091.071.081.071.081.071.081.071.101.031.091.081.051.071.081.071.081.051.08

Note: WBD = wet bulk density, DBD = dry bulk density, MAR = mass accumulation rate (g/cm2/1000 yr.), CAR = carbonate accumulation rate (g/cm2/1000 yr.), FFAR = fine fraction accumulation rate (g/cm2/1000 yr.).

523

J. C. ZACHOS ET AL.

65.0

66.0

67.0

1

i

(

)

Total CaC03 accumulation

Fine-fraction CaCOß^ accumulation

-* Insoluble fraction accumulation

}„\

i i0 0.5 1.0

Accumulation rate

(g/cm2/1000 yr.)

Figure 8. Total carbonate, fine-fraction carbonate, and insoluble frac-tion accumulation rates prior to and following the Cretaceous/Ter-tiary boundary.

Nonetheless, overall δ 1 8 θ values in uncontaminated Da-nian sections at Hole 577 tend to be lighter than Maes-trichtian values. Because preservation changes relativelylittle, this implies a slight warming in surface waters. Aslight cooling was apparently recorded by the whole rockvalues at Site 47.2 and by planktonic foraminifers fromSite 465 where the δ 1 8 θ values of Danian foraminifersare somewhat heavier (Fig. 4) than the values of theirMaestrichtian counterparts. Isotopic paleotemperature es-timates for surface waters at the studied sites averagenear 15-18°C for the Maestrichtian and 14-18°C forthe Danian. These temperatures appear to be rather coolfor the estimated paleolatitude of about 15°N (Fig. 8)(Lancelot and Larson, 1975), but are in line with thosecalculated from other published data (Boersma andShackleton, 1981).

The benthic oxygen-isotope stratigraphy from Site 465and Hole 577 suggests that bottom water temperaturesdid not change systematically from the Maestrichtian tothe Danian. At Site 465, δ 1 8θ values of Bulimina showedvariations amounting to about 4°C in paleotemperatureestimates. The range of δ 1 8 θ values of Aragonia in Hole577 was somewhat less. Because different species of ben-thic foraminifers were analyzed at each location, we canonly speculate on the relative bottom-water temperaturesof these two sites. However, considering the relativelysmall difference (0.5‰) in δ 1 8 θ between these two sec-tions, it may be reasonable to assume that bottom-watertemperatures at Sites 465 and 577 were very similar ifnot equal. The estimated isotopic paleotemperatures ofdeep water at the sites are between 8 and 12°C bothabove and below the Cretaceous/Tertiary boundary. Thesevalues fall within the range for shallow intermediate wa-ters (< 1500 m) determined for inoceramids for the Late

Cretaceous at other DSDP sites (Saltzman and Barron,1982). There appears to have been relatively little changein the vertical temperature gradient across the Cretaceous/Tertiary boundary at either Site 465 or 577, but δ 1 8 θ val-ues of Aragonia from Hole 577 suggest cooling acrossthe boundary whereas δ 1 8 θ values of Bulimina from Site465 apparently record a brief warming episode. The esti-mated paleodepths of Sites 465 and 577 are about 1500and 2400 m, respectively (using subsidence curves andassumptions of Thiede et al., 1982). The estimated pa-leotemperatures at Hole 577 appear to be too warm forthe deeper paleodepth.

The negative δ13C shift of about 1,0‰ recorded in thefine-fraction carbonate at the Cretaceous/Tertiary bound-ary at Hole 577 has been found in varying magnitudesat nearly every Cretaceous/Tertiary boundary sequencestudied (e.g., Arthur et al., 1979; Scholle and Arthur,1980; Hsü et al., 1982a; Perch-Nielsen et al., 1982). Thenegative shift detected in our data from Hole 577 occurswithin a few centimeters of the Cretaceous/Tertiaryboundary determined by calcareous nannofossil strati-graphy (Monechi, this volume) and the major iridiumspike and highest Ir/Fe ratio (Michel et al., this volume).The close correspondence of the δ13C anomaly and theiridium anomaly in most other sections (Hsü et al., 1982a;Zachos et al., unpubl. data) across the Cretaceous/Ter-tiary boundary suggest that the δ13C shift is an excellentstratigraphic marker for the boundary (barring signifi-cant hiatuses). Some investigators have interpreted thisanomaly to represent a decrease in oceanic surface waterphytoplankton production (Boersma et al., 1979; Boers-ma and Shackleton, 1981; Arthur and Dean, 1982; Hsüet al., 1982a, b). While a number of authors have specu-lated on the cause of the catastrophy that brought aboutthe extinctions and what effects the event and accompa-nying phytoplankton extinctions may have had on globalclimate, few have attempted to quantify the magnitudeof the decline in marine productivity or its duration.Boersma and Shackleton (1981) and Hsü et al. (1982a)determined the δ13C of benthic foraminifers as well asmeasuring the δ13C of corresponding planktonic orga-nisms, recognizing that the abrupt decline in δ13C acrossthe boundary might only have been a surface-water phe-nomenon. Benthic foraminifers tend to record the δ13Cof deep water (Woodruff et al., 1980, Belanger et al.,1981; Savin and Yeh, 1981) although, as mentioned ear-lier, some interspecies fractionation effects exist. By com-paring the benthic and planktonic foraminifer carbon-isotope records, Boersma and Shackleton (1981) wereable to establish rough estimates of surface to deep-wa-ter carbon-isotope gradients prior to and following theCretaceous/Tertiary boundary. Surface to deep-water car-bon-isotope gradients of the modern oceans have beencorrelated with nutrient gradients (Broecker, 1974, Broeck-er and Peng, 1982) and dissolved oxygen or apparent ox-ygen utilization (Kroopnick et al., 1970; Williams et al.,1977; Kroopnick, 1974, 1980) and, hence, with primaryproductivity in surface waters and carbon oxidation indeep-water masses. Therefore, variations in primary pro-ductivity and consequent transfer and oxidation of or-ganic matter in deeper waters should be reflected in

524


Figure 9. Correlation between Holes 577 and 577B showing magnetic stratigraphy (Bleil, this volume)and iridium concentrations as measured in Hole 577B (Michel et al., this volume). Cretaceous/Ter-tiary boundary was chosen at both holes on the basis of calcareous nannofossil biostratigraphy (Mo-nechi, this volume). The nannoplankton and planktonic foraminifer stratigraphies in the right twocolumns are those of Hole 577.

changes in the surface to deep water δ1 3C gradients in-ferred from calcareous planktonic and benthic organisms.From the Maestrichtian to the Danian at Site 465, therewas a decrease in the surface to deep water δ1 3C gradientfrom 1.5 to O‰ which was interpreted to represent an al-most complete cessation of total dissolved CO2 utiliza-

tion for organic carbon production in surface waters(Boersma and Shackleton, 1981). The change in Maes-trichtian to Danian surface- to deep-water carbon-iso-tope gradients at Hole 577, as the difference between theδ1 3C of fine-fraction carbonate and of the benthic fora-minifer Aragonia, was similar in magnitude to the change

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measured at Site 465 (Fig. 4). The decrease in the δ13Cgradient of 1.5%o at Site 465, however, was a direct resultof the decrease in planktonic carbon-isotope ratios be-cause the carbon-isotope values of the benthic foramini-fer Bulimina remained more or less constant. The changein surface to deep gradients at Hole 577 was partially afunction of both an approximate 0.8%o enrichment inbenthic carbon-isotope values and a decrease of 0.8 tol.O‰ in fine-fraction δ13C. This reduction of the surfaceto deep δ13C gradient at the Cretaceous/Tertiary bound-ary is similar in some respects to the reduction of theδ13C gradients in some freshwater lakes during the sum-mer to winter transition when surface water productivitydeclines and summer thermal stratification is removedby vertical mixing (e.g., McKenzie, 1982). Following afairly abrupt decline in surface organic productivity atthe Cretaceous/Tertiary boundary that might have ac-companied extinction of the majority of calcareous nan-noplankton species, the flux of organic matter to deep-water masses would rapidly decrease. The amount ofisotopically light CO2 released from the decaying detri-tus would therefore decrease and δ13C values for Daniandeep-water masses may be more positive relative to thoseof the Late Cretaceous.

Further upcore at Hole 577, benthic δ13C values de-crease, reaching preboundary levels within the Crucipla-colithus tenuis zone. In addition, surface- to deep-car-bon isotope gradients also return to preboundary levelsby 2 m.y. following the event. If we assume that a posi-tive surface to deep δ13C gradient is produced by nor-mally operating transfer mechanisms, such as the pre-ferred extraction of isotopically light carbon from sur-face water in the organic flux to deep waters resultingfrom higher primary productivity, then these data maybe interpreted to suggest that primary production recov-ered to near preboundary levels about 2 m.y. after theextinction event. However, the equilibrium levels of bothsurface and deep δ13C were apparently much lighter inthe Danian than in the Maestrichtian, perhaps becausethe overall rate of organic carbon burial was still some-what lower in the Danian than during the late Maes-trichtian (e.g., Scholle and Arthur, 1980) following theCretaceous/Tertiary "event." The lighter average δ13Cvalues would therefore be a whole-reservoir signal.

On the basis of the change in planktonic foraminifer/nannofossil ratios, it appears that the rate of productionof calcareous nannofossils decreased relative to the pro-duction of planktonic foraminifers or that planktonicforaminifers were able to recover much more quickly.Berger (1976), however, suggested that increases in theforaminifer/nannofossil ratio were the result of increasedwinnowing. This seems unlikely given the thickness ofthe section with higher foraminifer ratios (200 cm) andthe absence of apparent current-induced laminations orother primary sedimentary structures indicative of cur-rent action. Foraminifers have a greater depth stratifica-tion than nannofossils, which generally inhabit only thephotic zone, and as a group may have been able to main-tain higher levels of productivity. The transition from nan-nofossil to relatively foraminifer-rich sediment occurs gra-dually across the Cretaceous/Tertiary boundary. Coarse

fraction percentages increase from less than 1% to asmuch as 24% of the total sediment weight over a 60-cminterval above the boundary (Table 1). This suggests thatthe inferred productivity decrease may not have occurredinstantly, geologically speaking, or that bioturbationmixed out the originally sharp change (e.g., Thiersteinand Okada, 1979). Perch-Neilsen et al. (1982) and Hsüet al. (1982a) have presented evidence indicating that theextinction of calcareous nannoplankton may have oc-curred over a longer period than previously believed. How-ever, in light of the sudden shift in carbon-isotope rec-ords across the boundary in the apparently continuouspelagic sequence at Site 577, it would be difficult to sug-gest that the decline in productivity was a slow and grad-ual process. It must have occurred in a period of lessthan 28,000 yr.

CONCLUSIONSOn the basis of available biostratigraphic and geo-

chemical evidence (Monechi, this volume; Michel et al.,this volume), we assume that the pelagic carbonate se-quence at Hole 577 is continuous across the Cretaceous/Tertiary boundary. Our trace-element and SEM studiesof closely spaced samples across the Cretaceous/Tertiaryboundary suggest that there are relatively small changesin preservation of calcareous microplankton and nanno-plankton assemblages across the boundary. Oxygen-iso-tope determinations of the calcareous fine-fraction (rep-resenting surface-water masses) and benthic foramini-fers in Hole 577 suggest relatively little temperature changein surface or deep-water masses across the boundary. Si-milar results were obtained in studies of DSDP Site 47.2on the Shatsky Rise and Site 465 on nearby Hess Rise.The δ13C gradient, inferred from analyses of planktonicand benthic calcareous groups, decreased abruptly at theCretaceous/Tertiary boundary in conjunction with nan-nofossil and foraminifer extinctions and an increase inIr concentrations and the Ir/Fe ratio. We attribute thechange in the δ13C gradient to a sudden decrease in sur-face productivity associated with the extinction event.Further substantiation of this is given by a significantreduction of carbonate accumulation rates across the Cre-taceous/Tertiary boundary at Hole 577 and other locali-ties without apparent significant changes in carbonatepreservation. The decline in carbonate production/ac-cumulation apparently lasted at least 2 m.y. The rela-tionship to a postulated meteoritic impact, however, isnot clear. Previously hypothesized temperature and car-bonate preservational changes at the boundary are notevident at Pacific DSDP sites analyzed so far. The mech-anism^) for the extinctions and for decreased planktonproductivity at the Cretaceous/Tertiary boundary re-main^) obscure.

ACKNOWLEDGMENTS

The authors would like to thank Hans Thierstein, Nick Shackle-ton, James Kennett, and Audrey Wright for reviewing this manuscriptand offering helpful criticism and Maria Burdett for drafting originalfigures. This work was supported by NSF Grant EAR-8306561.

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Date of Initial Receipt: 17 January 1984Date of Acceptance: 17 August 1984

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Plate 1. Site 47.2. 1. Globorotalia trinidadensis (Bolli), Sample 47.2-11-2, 45 cm. Bar = 100 µm. 2. Danian calcareous nannofossil ooze, Sam-ple 47.2-11-2, 45 cm. Bar = 10µm. 3, 4. Maestrichtian calcareous nannofossils, Sample 47.2-12-1, 28 cm. Bar = 10µm. 5, 6. Spiral and um-bilical views of Globotruncana contusa (Cushman), Sample 47.2-11-1, 28 cm. Bar = 100 µm.

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™ - 1 -

r‰ , t• • • * /

* * l

- .ΛK *

*- *" ^ ^

S v hi. • Λ?

• ^ i

Plate 2. Site 577. 1. Aragonia (Finlay), Sample 577-12-5, 80 cm. Bar = 100 µm. 2. Bulimina (Cushman), Sample 577-12-5, 80 cm. Bar 100µm. 3, 4. Aragonia (Finlay), Samples 577-12-5, 80 cm and 577-12-5, 128 cm. Bar = 100 µm. 5, 6. Contrasting surface textures of two Arago-nia, Sample 577-12-5, 128 cm. Bar = 10 µm.

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Plate 3. (Bar represents 10 µm.) 1-4. Moderate to well preserved Danian calcareous nannofossil assemblage. Note the lack of overgrowths andetchings on the coccolith shields, Sample 577-12-2, 50 cm. 5, 6. Coccoliths and coccolith fragments from Zone NP2. Calcite overgrowths arepresent on some specimens, Sample 577-12-5, 80 cm. 7. Surface of a planktonic foraminifer from the boundary, sample 577-12-5, 125 cm. 8.Coccolith ooze with foraminifer test in background, Sample 577-12-5, 125 cm.

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Plate 4. (Bar represents 10 µm.) 1-4. Various calcareous nannofossils from boundary samples. Most specimens are moderately to well preserved.Overgrowths can be seen on several coccolith shields. Sample 577-12-5, 128 cm. 5-8. Coccolith ooze of Maestrichtian age. Euhedral calciteovergrowths are present on most coccolith specimens. Partially disaggregated shields and fragments are also present.

532