Organic Geochemistry 80 (2015) 1–7

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Organic Geochemistry

journal homepage: www.elsevier .com/locate /orggeochem

Scarcity of the C30 sterane biomarker, 24-n-propylcholestane, in LowerPaleozoic marine paleoenvironments

http://dx.doi.org/10.1016/j.orggeochem.2014.11.0080146-6380/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Organic Geochemistry Unit, School of Chemistry, TheCabot Institute, University of Bristol, BS8 1TS, UK. Tel.: +44 79 6088 1592.

E-mail addresses: mrohr001@ucr.edu (M. Rohrssen), bcgill@vt.edu (B.C. Gill),glove@ucr.edu (G.D. Love).

Megan Rohrssen a,⇑, Benjamin C. Gill b, Gordon D. Love a

a Department of Earth Sciences, University of California, Riverside, CA 92521, USAb Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 August 2014Received in revised form 10 November 2014Accepted 22 November 2014Available online 3 December 2014

Keywords:24-n-PropylcholestanePelagophytePaleozoic

24-n-Propylcholestane (24-npc), a C30 sterane compound derived from sterol precursors which are themajor sterol constituents of modern pelagophyte microalgae, occurs in certain Neoproterozoic rocksand oils and throughout the Phanerozoic rock record. This broad distribution leads 24-npc to be widelyconsidered a reliable indicator of open to partially restricted marine depositional conditions for sourcerocks and oils. Here we report two significant hiatuses in the occurrences of 24-npc in the Lower Paleo-zoic marine rock record: the first in the Middle–Late Cambrian and the second in the Late Ordovician–early Silurian transition for a range of lithofacies (carbonates and siliciclastic rocks), organic carbon con-tents (both organic-lean and organic-rich), and paleoceanographic environments (shelf and deeper watermarine settings) and observed offshore of two paleocontinents, Laurentia and Baltica. The Ordovician–Silurian gap is at least 9 million years, and possibly up to 20 million years, in duration. Robust olderoccurrences of 24-npc steranes in some Neoproterozoic rocks and oils suggest that oceanographic condi-tions in our intervals of Lower Paleozoic time were unfavorable for the proliferation of pelagophyte algaeas phytoplankton. Caution should therefore be applied when interpreting a lacustrine versus marinedepositional environmental setting for source rocks and oils in these intervals of Early Paleozoic timeusing lipid biomarker assemblages.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

While paleoenvironmental assessment is best conducted usinga suite of lipid biomarker proxies, detection of the C30 sterane24-n-propylcholestane (24-npc) is commonly used as an indicatorof ancient marine sedimentary depositional environment(Moldowan, 1984; Moldowan et al., 1990). Typically present inmarine Phanerozoic rocks and oils at abundances of 3–10% of totalsteranes (e.g. McCaffrey et al., 1994), in rare cases 24-npc may con-stitute as much as 20–53% of total steranes (Killops et al., 2000).The absence of 24-npc in Lower Paleozoic source rocks and oilsindependently constrained to marine depositional conditions wasinitially used to infer that the eukaryotic source organisms hadnot evolved until after the Ordovician (Moldowan, 1984;Moldowan et al., 1990). Subsequent work has unambiguously

identified 24-npc as indigenous in marine rocks throughout thePhanerozoic, extending to samples as old as Neoproterozoic inage (e.g. McCaffrey et al., 1994; Grosjean et al., 2009; Love et al.,2009; Kelly et al., 2011).

24-n-Propylidene-cholesterol and 24-n-propylcholesterol, theprecursor molecules to 24-npc, have at present only been identi-fied in pelagophyte microalgae (Volkman, 2003; Giner et al.,2009), and in a culture of the heterotrophic foraminifer Allogromialaticollaris (Grabenstatter et al., 2013). Pelagophyceae is a smallclass of marine chromophyte algae which synthesize 24(E)-24-pro-pylidene-cholesterol as their dominant sterol through an unusualpathway involving a cyclopropyl intermediate (Giner andDjerassi, 1991; Hong et al., 2013). Molecular clock analyses indi-cate a Paleozoic to Neoproterozoic appearance of Pelagophyceae(e.g. Brown and Sorhannus, 2010), which broadly agrees with thesecular record of 24-npc in sedimentary rocks and oils. Here wereport evidence for hiatuses in the 24-npc biomarker record inextracts of thermally immature marine sedimentary rocks duringtwo intervals of the Lower Paleozoic era, findings which may beof use in oil-source rock correlation.

2 M. Rohrssen et al. / Organic Geochemistry 80 (2015) 1–7

2. Materials and methods

2.1. Materials and geological context

The dataset comprises lipid biomarkers extracted from two dis-tinct sets of rock samples of different ages. The first set is a suite ofMiddle–Late Cambrian carbonate rocks collected from drill core(Gill et al., 2011). The second consists of Late Ordovician to earlySilurian age carbonate and siliciclastic marine rocks obtained fromoutcrop and drill core. All of the sedimentary rocks analyzed werewithin the pre-oil window to peak-oil window thermal maturityrange based on molecular organic geochemical (Table 1) as wellas independent thermal maturity proxies, and ideal for lipid bio-marker investigation.

The first suite of samples derives from Cambrian (Mindyallen,Idamean and Iverian Australian regional stages, Guzhangian,Paibian and Jiangshanian International stages, �500.5–489.5 MaGTS2012) rocks of the Georgina Limestone Formation, Queensland,Australia. The Mount Whelan #1 core was drilled on the EasternGeorgina Basin on the margin of the Toko Syncline and did notreach the higher level of thermal maturity observed in the deeperwestern portion of the basin (Ambrose et al., 2001). Geochemicalinvestigation of the Georgina Limestone in the Mount Whelan #1core indicates that the formation has a fair to very good source rat-ing and is within the oil window (Jackson, 1982). The GeorginaLimestone at this location (�90%) consists almost entirely of argil-laceous micrite, which is laminated on the millimeter scale (Greenand Balfe, 1980). Occasional decimeter scale upward-fining unitswith peloidal, intraclastic and oolitic packstones or grainstonesas bases likely represent turbidite deposits. Due to the presenceof the fine lamination and a general lack of wave current structuresthe depositional environment of the Georgina has been interpretedas below storm wave base (Green and Balfe, 1980). The age of theGeorgina Limestone is constrained by trilobite biostratigraphy andidentification of a positive carbon and sulfur isotope excursion cor-related to the globally recognized Steptoean Positive Carbon Iso-tope Excursion (‘SPICE,’ Saltzman et al., 2000; Gill et al., 2011).The occurrence of globally distributed trilobite fauna and the SPICEin the studied core and elsewhere indicates a marine environmentfor deposition of these rocks.

The second suite of rocks comes from the Late Ordovician–earlySilurian marine rock successions of the Laurentian paleocontinent:Anticosti Island, Quebec, Canada; the Vinini Formation, Nevada,USA; and Maquoketa Formation, Iowa, USA. These rock units spana range of marine conditions including shallow epeiric and shelfaldeposits and upwelling-influenced epeiric and marginal rocks(Table 1) (Raatz and Ludvigson, 1996; Finney et al., 1999; Long,2007; Desrochers et al., 2010). In addition to biostratigraphic ageconstraints, a substantial positive d13C isotopic excursion of approx-imately 4‰, representing the globally recorded Hirnantian carbonisotope excursion, indicates a Hirnantian age (445.2–443.4 MaGTS2012) for rocks on Anticosti and the Vinini Formation (LaPorteet al., 2009; Jones et al., 2011).

Anticosti Island preserves mixed carbonate-siliciclastic rocksdeposited in a tropical, storm influenced ramp to shelf environ-ment. Samples were collected from outcrops of the Vauréal, EllisBay, Becscie and Merrimack formations on western Anticosti(Rohrssen et al., 2013) and drill core material (D3) from the Macas-ty Formation. High subsidence rates coupled with high eustatic sealevel are thought to have kept the Anticosti Basin in contact withthe open ocean and the Laurentian epeiric seaway through muchof the Late Ordovician (Long, 2007). Shallower intervals duringdeposition of the Vauréal and upper Ellis Bay Formation due todecreased subsidence and Hirnantian Stage regression are indi-cated by development of coral–algal and coral–stromatoporoid

bioherms, often accompanied by oncoidal limestones (Copper,2001). Crinoid, trilobite and other marine fossils present in eventhe shallowest water settings, however, suggest that westernAnticosti depositional conditions remained marine (Copper,2001; Long, 2007; but see also Desrochers et al., 2010).

Vinini Formation samples derive from the Vinini Creek Section inthe Roberts Mountains of Nevada, USA (Finney et al., 1999). TheVinini Formation comprises interbedded graptolitic brown shaleswith sandstones and siltstones which, during the Hirnantian low-stand, are replaced with organic carbon rich carbonates (Finneyet al., 1997). Phosphatic intervals within the brown shale rich mid-dle of the section (Normalograptus pacificus zone) are interpreted asrecording deep water marine deposition (continental slope/basinalfacies) under an oxygen minimum zone (Finney et al., 1999, 2007).Shallowing in the upper N. pacificus zone continues through theHirnantian Metabolograptus extraordinarious and Metabolograptuspersculptus graptolite biozones but did not result in exposure of thisdeep water section (Finney et al., 1997). A low angle thrust faultcross cuts the upper portion of the Vinini Creek section resultingin anomalous biomarker maturity indicators proximal to the faultpossibly due to frictional heating (c.f. Polissar et al., 2011) but bio-stratigraphic constraints indicate that the fault does not appear tohave caused duplication or loss of section (Finney et al., 1997), orhave introduced allochthonous bitumen.

The Late Ordovician-age Maquoketa Formation is roughly con-temporaneous with the Vauréal Formation of Anticosti Island,and with pre-Hirnantian Vinini Formation strata (Sadler et al.,2009), although putative Hirnantian Stage strata are poorly pre-served and could not be obtained in core or outcrop for this study.Phosphatic, pyritic, organic carbon rich (Table 1) and cherty sedi-ments rocks in the basal Elgin Member have been attributed toupwelling of nutrients derived from the Sebree Trough (e.g.Witzke, 1987; Raatz and Ludvigson, 1996). In Iowa, Illinois, andIndiana the Maquoketa Formation has been examined for oil-source rock correlation and petroleum potential (e.g. Guthrie,1996). Maquoketa Formation shales (Scales and Elgin members)in the nearby Cominco SS4A core have Rock-Eval Tmax of up to441 �C, hydrogen indices of 278–1024 mg HC/g TOC (Guthrie,1996). Samples were collected from the Big Springs 5 core (BS-5)and capture basal Elgin Member phosphorite and brown shale aswell as the overlying gray–green shales and dolomitized carbon-ates of the Elgin, Clermont, Fort Atkinson and Brainard members(Table 1). BS-5 was drilled in Clayton County in 1989 (Iowa Geolog-ical and Water Survey GEOSAM well number 30190) and is in thenorth-central area described by Raatz and Ludvigson (1996). Forcomparison, an outcrop sample from the lower Elgin Memberwas collected from an organic rich shale interval immediatelybelow the main Isorthoceras nautiloid bed at the Graf Roadcut (Stop5, Witzke et al., 1997).

Finally, we also present data from carbonates and marls of theSilurian age (Telychian–Sheinwoodian; 438.5–430.5 Ma GTS2012)Visby Formation of Gotland from offshore the paleocontinent ofBaltica. Visby Formation sediments comprise distal shelf deposits,with alternating limestones and marls of the Lower VisbyFormation grading up-section into the carbonate dominated, bio-herm bearing Upper Visby Formation (Calner et al., 2004) similarto lithofacies on Anticosti Island. Visby Formation rocks capturethe Sheinwoodian positive carbon isotope excursion (Azmy et al.,1998; Cramer and Saltzman, 2005) and elevated extinction inter-vals of the Ireviken Event (e.g. Munnecke et al., 2003), facilitatingtemporal constraints. Lower and Upper Visby Formation rocks alsohave very low thermal maturities, identified both on the basis ofconodont alteration indices of 1 (Jeppsson, 1983) and clumped iso-tope paleothermometry measurements which identify a maximumof approximately 60 �C during burial (Cummins et al., 2014).

Table 1Selected lipid biomarker ratios and abundances from MRM–GC–MS analysis of Lower Palaeozoic outcrop and core samples.

Sample Formation Locationa Ageb Lithology TIC(wt%)

TOC(wt%)

C29 sterane dia/(dia + regular)

C29 sterane aaaS/(aaaS + aaaR)

Hopane/steranec

C29 St(%)d

C30 St(%)d

All steranes(ppm TOC)

C30 steranes(ppm TOC)

Standard JR13 sna, Jet Rock UK J Shale n.m. n.m. 0.85 0.48 0.48 39.3 4.9 n.a. n.a.Standard AGSO Oil Standard n.a. n.a. n.a. n.a. n.a. 0.94 0.51 2.00 40.9 3.3 n.a. n.a.Standard North Sea oil (n = 9) UK J n.a. n.a. n.a. 1.49 ± 0.14 0.52 ± 0.03 1.19 ± 0.20 33.3 ± 1.4 8.3 ± 0.1 n.a. n.a.901-16.0 Becscie Anticosti, Canada S–R Carbonate 10.6 0.2 0.19 0.48 5.53 65.3 0.1 6.9 < 0.1901-8.0 Ellis Bay Anticosti, Canada O–H Arg. carbonate 7.2 0.2 0.49 0.49 2.51 60.5 0.4 19.4 0.1901-6.4 Ellis Bay (n = 3) Anticosti, Canada O–H Arg. carbonate 8.6 < 0.1 0.48 ± 0.01 0.48 ± 0.01 0.34 ± 0.01 61.7 ± 0.6 0.3 ± 0.0 2.1 ± 0.0 < 0.1906-44 Vauréal Anticosti, Canada O–K Carbonate 9.4 0.4 0.57 0.45 4.12 52.2 0.2 30.6 < 0.1D3-3927 Macasty, D3 core Anticosti, Canada O–K Shale 1.6 4.9 0.69 0.54 0.35 54.6 0.7 6.5 < 0.1D3-4002 Trenton-Black River, D3 core Anticosti, Canada O–Sa/K Arg. carbonate 5.3 0.4 1.13 0.56 0.72 42.4 1.2 1.3 < 0.1SGH-TOS Whitewater/Liberty Indiana, USA O–K Carbonate 11.5 0.2 0.37 0.23 3.11 37.5 0.1 0.3 < 0.1SGH-43.0 Liberty/Whitewater Indiana, USA O–K Arg. carbonate 1.5 0.2 0.15 0.21 6.22 51.1 0.1 3.4 < 0.1SGH-13A Liberty Indiana, USA O–K Arg. carbonate 5.5 0.2 0.47 0.12 1.86 46.7 0.1 6.9 < 0.1VC-08 Vinini, 23.8 m Nevada, USA O–H Carbonate 11.2 0.5 0.20 0.27 0.19 70.1 0.2 67.5 0.1VC-01 Vinini, 20.0 m Nevada, USA O–H Arg. carbonate 8.1 0.5 0.16 0.29 0.20 69.2 0.2 596.9 1.0VA-23 Vinini, 11.4 m Nevada, USA O–K Shale < 0.1 18.4 0.24 0.50 0.72 60.6 0.3 7.6 < 0.1VA-20 Vinini, 9.6 m Nevada, USA O–K Carbonate 10.3 2.1 0.28 0.50 0.53 70.7 0.1 4.3 < 0.1VA-08 Vinini, 2.1 m Nevada, USA O–K Carbonate 9.6 0.5 0.22 0.39 0.69 64.4 0.1 21.0 < 0.1BS-5 146.4 Maquoketa, BS-5 core, 146.4 ft Iowa, USA O–K Arg. carbonate 5.8 0.2 0.48 0.06 0.57 57.0 0.0 34.6 < 0.1BS5-183.5 Maquoketa, BS-5 core, 183.5 ft Iowa, USA O–K Arg. carbonate 8.2 < 0.1 0.39 0.10 0.37 53.4 0.1 95.4 0.1BS5-202.5 Maquoketa, BS-5 core, 202.5 ft Iowa, USA O–K Arg. carbonate 6.8 0.5 0.32 0.07 0.38 51.0 0.1 47.3 < 0.1BS-5-212.9 Maquoketa, BS-5 core, 212.9 ft Iowa, USA O–K Arg. carbonate 6.9 0.8 0.55 0.08 0.27 52.3 0.1 35.8 < 0.1BS5-221.1 Maquoketa, BS-5 core, 221.1 ft Iowa, USA O–K Arg. carbonate 7.7 0.5 0.50 0.10 0.47 62.1 0.0 83.4 < 0.1BS5-280.3 Maquoketa, BS-5 core, 280.3 ft Iowa, USA O–K Arg. carbonate 2.5 5.1 0.43 0.10 0.37 51.5 0.1 49.5 0.1BS5-288 Maquoketa, BS-5 core, 288 ft Iowa, USA O–K Shale 0.5 9.0 0.40 0.10 0.11 53.2 0.1 44.6 < 0.1BS5-298hg Maquoketa, BS-5 core, 298 ft Iowa, USA O–K Carbonate 10.2 1.4 0.20 0.12 0.59 51.4 0.1 3.6 < 0.1Graf Maquoketa, Graf outcrop Iowa, USA O–K Shale 0.5 12.8 0.55 0.20 0.26 53.9 0.1 n.m. n.m.Boda+25 Kallholn Siljan, Sweden S–R Shale 1.2 6.0 0.38 0.31 0.29 67.2 0.2 n.m. n.m.G1+6.0 Visby Gotland, Sweden S–W Marl 10.9 0.3 0.55 0.12 1.13 65.6 0.3 5.0 < 0.1G1+2.0 Visby Gotland, Sweden S–W Marl 9.9 0.2 0.56 0.12 0.92 65.1 0.6 12.4 0.1G1+1.0 Visby Gotland, Sweden S–W Marl 8.5 < 0.1 0.63 0.12 1.05 65.0 0.8 143.6 1.2WHI-6A Whelan core 35.8 m Australia C–St Arg. carbonate n.m. n.m. 3.14 0.59 0.74 47.0 0.3 n.m. n.m.WHI-38 Whelan core 135.7 m Australia C–St Arg. carbonate n.m. n.m. 2.26 0.53 0.20 46.7 0.3 n.m. n.m.WHI-62 Whelan core 210.3 m Australia C–St Arg. carbonate n.m. n.m. 3.58 0.56 0.13 42.4 0.6 n.m. n.m.WHI-92 Whelan core 302.1 m Australia C–St Arg. carbonate n.m. n.m. 0.65 0.52 1.36 32.8 1.8 n.m. n.m.

a Sample sites and nomenclature for Anticosti Island and the Vinini Formation as described in Rohrssen et al. (2013), and for Whelan Formation as in Gill et al. (2011).b Abbreviations: C, Cambrian; Ca, Caradoc; D, Darriwillian; H, Hirnantian; J, Jurassic; K, Katian; O, Ordovician; R, Rhuddanian; S, Silurian; S, Steptoean; W, Wenlock; TIC, Total Inorganic Carbon; TOC, Total Organic Carbon; n.a.,

not applicable; n.m., not measured; arg. argillaceous.c Hopane/sterane calculated from (C27–C35 hopanes)/(C27–C29) steranes, measured in MRM–GC–MS.d Expressed as total of C27–C30 steranes.

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4 M. Rohrssen et al. / Organic Geochemistry 80 (2015) 1–7

2.2. Methods

Outcrop and core samples (Table 1) for biomarker analysis weretrimmed with a water-cooled rock saw to remove outer weatheredsurfaces and to expose a solid inner portion and sonicated in asequence of ultrapure water, methanol, dichloromethane (DCM)and hexane before a final rinse with DCM. Clean rock pieces werewrapped thoroughly in aluminium foil (heated at 550 �C over-night), before crushing with a hammer. Sample fragments werepowdered in a ceramic puck mill (SPEX) cleaned between samplesby powdering two batches of fired sand (850 �C overnight) andrinsing with the above series of solvents. Lipid biomarkers wereextracted in 9:1 v:v DCM:methanol using a Microwave AcceleratedReaction System (CEM Corp.) and separated into aliphatic, aro-matic and polar fractions by silica column chromatography. Satu-rated hydrocarbon fractions were analyzed by metastablereaction monitoring–gas chromatography–mass spectrometry(MRM–GC–MS, Fig. 1) conducted on a Waters Autospec Premiermass spectrometer equipped with an Agilent 7890A gas chromato-graph and DB-1MS coated capillary column (60 m � 0.25 mm,0.25 lm film) using He for carrier gas. The GC temperature pro-gram consisted of an initial hold at 60 �C for 2 min, heating to150 �C at 10 �C/min followed by heating to 315 �C at 3 �C/min,and a final hold at 320 �C for 22 min. Isomers of 24-npc wereunambiguously identified when present in MRM–GC–MS bycomparison of retention time difference between C29 steranesand 24-npc diastereoisomers determined in a known oil standard.A deuterated C29 sterane standard (d4-aaa-24-ethylcholestane(20R), Chiron Laboratories AS) was added to saturated hydrocarbonfractions prior to MRM–GC–MS to quantify biomarker peaks, with50 ng internal standard added to the saturate hydrocarbonfraction. In MRM analyses, this standard compound was detectedusing the m/z 404 ? 221 transition. Procedural blanks typicallyyielded < 0.5 ng/g of powder of individual hopane isomersand < 0.3 ng/g powder for the most abundant steranes. Cross-talkof non-sterane signal in m/z 414 > 217 from C30 and C31 hopanesis < 0.2% of m/z 412 ? 191 hopane signal and < 1% of the m/z426 ? 191 signal, respectively.

Rel

ativ

e In

tens

ity

North Sea oil standard11Feb03_33

αααRβαR

αββR

,S

C : 372>21741%

27

C : 386>21714%

28

C : 400>217100%

29

Silurian Becscie Fm901-16.0,11Feb03_43

C : 414>2172%

30

C : 414>21730

αααRαααS

βαS βαR

αββR,S

51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

αααSβαS

xC αβH30

αααRαααS

βαS βαR

αββR,S

αααRαααSβαS

βαR

αββR,S

Retention Time (minutes)

Fig. 1. MRM–GC–MS ion chromatograms of extractable C27–C30 steranes from anearly Silurian marl from the Becscie Formation, Anticosti Island. Percentagesindicate the relative signal intensity for each sterane channel, with a C29 steranedominance typical for marine rocks and oils from the Early Paleozoic. Note the verylow abundance of C30 steranes in the m/z 414 ? 217 trace, approaching detectionlimits and overwhelmed by cross-talk from the C30 hopane channel (m/z412 ? 191). The C30 sterane trace for the North Sea oil standard is provided forcomparison.

Absolute and relative abundances of C30 desmethylsteranes areso low in these samples that cross-talk and co-elution often makeinterfering compounds the most intense peaks in the m/z414 ? 217 MRM ion chromatograms (Fig. 2), even though thecross-talk corresponds to only a small fraction of the interferingcompound’s signal. The major hopane and sterane compoundsfrom our rocks gave abundant signal in MRM–GC–MS analyses,so the absence of 24-npc steranes is not due to a lack of signal orpoor signal/noise. Often, a peak in the m/z 414 ? 217 trace thatappears to correspond to one of the 24-npc isomers is not accom-panied by the other isomers, suggesting that it does not, in fact,correspond to 24-npc.

Given the inherent inaccuracies of measurements close todetection limits of 24-npc, we applied a highly conservative metricfor detection by integrating background noise at the appropriateretention times in addition to any real 24-npc signal. Integrating‘noise’ in C30 sterane-free standards results in constant low abso-lute abundances of 0.05 ± 0.02 ng (n = 5) using MRM–GC–MS. Asa result, particularly when overall signal is relatively low as inthe Cambrian-age Georgina Limestone Formation samples, ourdetection metric overestimates the abundance of C30 steranes(Fig. 2, Table 1). Our reported C30 sterane percentages (Table 1)are therefore maximum estimates: the real abundance givenhigher sensitivity and selectivity will likely be smaller. It is impor-tant to note that any lack of 24-npc sterane signal is not due to lackof signal, even for the most organic lean samples, since C26–C29

sterane signal was abundant in all our MRM–GC–MS analyses.

3. Results and discussion

In the Middle–Late Cambrian age Georgina Limestone Forma-tion, 24-npc is below detection limits throughout sampled corecoverage despite there being independent paleobiological andchemostratigraphic indicators of marine deposition (Fig. 2A,Table 1) and constrained at representing < 1.3% of C27–C30 regularsterane compounds. The absence of detectable 24-npc in the Geor-gina Limestone Formation continues through a period exhibitingan expansion and then a reduction in the extent of widespreadmarine euxinia and oxygen minimum zones (Gill et al., 2011), dur-ing which substantial perturbations to the oceanic nitrogen cyclemay be expected (Higgins et al., 2012).

In Late Ordovician-age Anticosti rock extracts (Table 1, Figs. 1and 2B) 24-npc isomers are only present in trace amounts in drillcore from the middle Katian age (�448 Ma) Macasty Formation(�0.7% of C27–C30 steranes) near Autospec MRM detection limits,despite abundant signal from C26–C29 steranes (e.g. Fig. 1). Middleand middle-upper Ordovician rock extracts and oils of the TarimBasin, China, which may be of similar age yield MRM–GC–MSdetectable levels of 24-npc (Zhang et al., 2000), however the tem-poral constraints are insufficient to assign precise stratigraphiccorrelation to samples presented here. It may be possible thatthe high ratios of 24-iso to 24-n-propylcholestanes observed inTarim Basin middle and middle-upper Ordovician rock extractsand oils relative to Cambrian rocks may be due to low proportionalabundance of 24-npc as much as to high amounts of 24-iso-propyl-cholestane. On Anticosti, 24-npc lies below the detection limit inyounger strata (the Rawtheyan through Rhuddanian age Vauréal,Ellis Bay, Becscie and Merrimack formations; Fig. 2B). Traceamounts of 24-npc may be present in the Trenton-Black River For-mation on Anticosti (Fig. 2) although not observed in these strataelsewhere (Obermajer et al., 1998). The disappearance of 24-npcbiomarkers from the sterane assemblage substantially precedessedimentological or paleobiological evidence for environmentalperturbations associated with the Hirnantian Stage glacial maxi-mum and does not detectably increase during an interval shown

A30

βαS

30 β

αR

30 α

ααS

30 α

ααR

30 α

ββR

, S

Cambrian (210.3 mcd)WHI 62 sats09April20_2110% 400>217

North Sea oil sats 09April20_1520% 400>217Cambrian (135.7 mcd)WHI 38 sats11Feb03_496% 400>217

B

Ordovician Macasty FmD3-3927 sats 1/50uL10Nov05_1354% 400>217

Ordovician Ellis Bay Fm901-8.0 sats10Jul13_272% 400>217

Silurian Becscie Fm901-16.0 sats 11Feb03_432% 400>217

Ordovician Vaureal Fm906-44 sats 09Nov24_052% 400>217

Ordovician Trenton-Black River, D3-4002 sats 13Jan10_0510% 400>217

North Sea oil sats 11Feb03_3321% 400>217

*

*

*

30 β

αS

30 β

αR

30 α

ααS

30 α

ααR

30 α

ββR

, S

29 α

ααS

29 α

ββ R

,S

29 α

ααR

Retention Time

30 α

βH D

North Sea oil sats 13Feb12_0120% 400>217

Ordovician Maquoketa FmBS-5 core, 183.6 feet sats13Feb12_02<1% 400>217

Ordovician Maquoketa FmBS-5 core, 212.9 feet sats13Feb12_08<1% 400>217

Ordovician Maquoketa FmBS-5 core, 298 feet sats13Feb12_03<1% 400>217

Retention Time

*

*

*

30 β

αS

30 β

αR

30 α

ααS

30 α

ααR

30 α

ββR

, S

29 α

ααS

29 α

ββ R

,S

29 α

ααR

30 α

βH

C

Ordovician Vinini Fm11.4 m sats11Nov14_161% 400>217

Ordovician Vinini Fm 23.8 m sats 11Nov14_148% 400>217

Ordovician Vinini Fm 2.1 m sats11Aug02_07<1% 400>217

North Sea oil sats11Aug02_0220% 400>217

*

*

30 β

αS

30 α

ααS

30 α

ααR

30 α

ββR

, S

29 α

ααS

29 α

ββ R

,S

29 α

ααR *

30 β

αR

30 α

βH

Fig. 2. MRM–GC–MS ion chromatograms (m/z 414 ? 217) for detection of C30 steranes for samples from: (A) the Georgina Limestone Formation; (B) Anticosti Island, Canadaand Indiana, USA; (C) the Vinini Formation at Vinini Creek, USA; and (D) the Maquoketa Formation, USA. Percent peak height intensity of the m/z 414 ? 217 (C30 sterane)chromatogram relative to m/z 400 ? 217 (C29 sterane) signal intensity are indicated. In most cases cross-talk from C29 steranes (m/z 400 ? 217, aaaR indicated by ⁄) is thehighest response in the C30 sterane channel as C30 sterane signal is so low. The calculated elution times of six 24-n-propylcholestane isomers are labeled for the oil standardand for cross-talk from four of the C29 sterane isomers.

M. Rohrssen et al. / Organic Geochemistry 80 (2015) 1–7 5

to have had higher overall algal contribution to sedimentaryorganic matter (Rohrssen et al., 2013). 24-npc isomers remainbelow MRM–GC–MS detection limits on Anticosti Island in thestratigraphically highest investigated sample (Merrimack Forma-tion, approximately 439 Ma, Fig. 2B), indicating scarcity of24-npc for at least 10 myr on Anticosti.

Despite higher total organic carbon and overall greater contri-butions from algae to sedimentary organic matter (Table 1) in bothmudstone and carbonate samples of the Vinini Formation at VininiCreek (Fig. 2C), in the basal portion of the Maquoketa Formation(Fig. 2D), 24-npc remains below GC–MS detection and near detec-tion limits of MRM–GC–MS. Hydrous pyrolysis products from theVinini Formation also evidently lack SIR–GC–MS-detectable

amounts of C30 steranes (Wei et al., 2007). GC–MS analyses of oilsand source rocks in southwestern Ontario, Canada found thatDevonian and Ordovician sources could be distinguished, in con-junction with other features, by detection of C30-desmethylster-anes in Devonian oils with Ordovician oils apparently lackingC30-desmethylsteranes (Obermajer et al., 1998).

Scarcity of 24-npc is also observed in the Llandovery age blackshale of the Kallholn Formation, Sweden (Table 1). The Lower Silu-rian Qusaiba oil of North Africa also appears to lack GC–MS detect-able levels of C30 steranes (Cole, 1994). In contrast, samples of theLlandovery–Wenlock age Visby Formation of Gotland do containtrace levels of 24-npc at 0.4–0.8% of total C27–C30 steranes (Table 1),despite low total organic carbon content (Table 1). Molecular

6 M. Rohrssen et al. / Organic Geochemistry 80 (2015) 1–7

organic geochemical maturity parameters, such as sterane isomer-ization (aaaS/(aaaS + aaaR), Table 1), indicate a range of pre-oilwindow to oil window thermal maturities (C29 S/(S + R) of 0.10–0.56); neither excessive thermal maturity nor immaturity may beresponsible for the low abundance of 24-npc isomers. The lowabundance of 24-npc is likely not attributable to low organic car-bon content (total organic carbon contents up to 18.4%, Table 1)or biomarker yields, and on both the Laurentain and Baltic paleo-continents. Unfortunately, unconformities truncate the Vinini(Finney et al., 1997) and Maquoketa formations (Witzke et al.,1997) such that identifying the next appearance of 24-npc isomersat these locations is not feasible. Thus, although many more oilsand source rocks remain to be examined by MRM–GC–MS, ourresults suggest that algae which produced precursors to 24-npcwere not abundant in the ocean in the Late Ordovician–earlySilurian transitional period, whether in oligotrophic, warm envi-ronments as epitomized by Anticosti Island or in nutrient-replete,algal-dominated environments represented by the Vinini Forma-tion at Vinini Creek. Late Ordovician–early Silurian (upper Katianthrough Rhuddanian stages) rocks from throughout North Americalack 24-npc in GC–MS detectable levels (< 0.5% of total steranes)and where potentially present in MRM–GC–MS remain near detec-tion limits (Fig. 2B and D), reflecting the ubiquity of scarce 24-npcin Late Ordovician through early Silurian age rocks. Further MRM–GC–MS analyses of Lower Paleozoic rock extracts and oils will helpconstrain when readily detectable 24-npc sterane signals reappearin Silurian time (at levels > 0.5% of total C27–C30 steranes).

The scarcity of 24-npc in intervals of the Lower Paleozoic is par-ticularly puzzling as modern pelagophyte algae are physiologicallyversatile and widespread in present day marine environments. Pel-agophyceae are present as phytoplankton in open marine, coastalmarine, estuarine and superlittoral modern settings, but wellknown for their tendency to form harmful algal blooms in boththe warm eutrophic waters and saline lagoons of the Gulf ofMexico and the cooler, less nutrient-replete waters of the NorthAtlantic (e.g. DeYoe et al., 1997; Rhudy et al., 1999; Gobler andSanudo-Wilhelmy, 2001; Lomas et al., 2001; Doblin et al., 2004;Berg et al., 2008; Worden et al., 2012). Aureococcus, for example,is able to grow on both organic and inorganic nitrogen species,though out-competed by other algae under nitrate-replete condi-tions, able to uptake organic phosphorous, and able to supplementphotosynthetic carbon fixation with assimilation of glucose(Gobler and Sanudo-Wilhelmy, 2001; Lomas et al., 2001; Berget al., 2008). Less is known about the metabolic versatility of theiropen ocean counterparts (Worden et al., 2012). C30 sterols in thepresent day ocean are sometimes not detected in nearshore envi-ronments, despite similar overall sterol yields to offshore settingswherein C30 sterols are observed (Hernández-Sánchez et al.,2014). In this case, lower abundances of C30 steranes correspondto diminished picoplankton cell counts, attributed to differencesin nutrient regime near versus offshore (Hernández-Sánchezet al., 2014). Further study of open ocean pelagophyte physiologywould greatly assist interpretation of the precise nutrient condi-tions (e.g. low total nitrogen) that favor pelagophytes. For this rea-son, it is important to note that the sites presented here encompassboth epeiric and shelf/slope deposits.

The time periods focused on in this study are all characterizedby having extended intervals with positive shifts in the carbon iso-tope composition of marine carbonates and organic matter andwith elevated macro- and microfaunal extinction rates, withsubstantially lower marine sulfate concentrations [possibly in the5–10 mM range (Brennan and Lowenstein, 2002; Horita et al.,2002)] compared with the modern ocean, and with changes inthe extent of oxygen minimum zones (e.g. Saltzman et al., 2000).The global and regional effects of expansion and contraction of

oxygen minimum zones upon nitrogen cycling (c.f. LaPorte et al.,2009; Higgins et al., 2012) may well play a role in the abundanceof pelgophytes. However, the ubiquitous and long duration ofscarce 24-npc in both organic rich (Vinini Formation, Elgin Memberof the Maquoketa Formation, Kallholn Formation) and organic lean(Anticosti rocks, upper members of the Maquoketa Formation) sed-imentary rocks, before and after recognized oceanographic andgreenhouse–icehouse–greenhouse climatic changes, obscurespotential linkages between these events and the scarcity of 24-npc in the studied locations. Irrespective of the mechanism(s)responsible, trace amounts of 24-n-propylcholestane is a usefulfeature for correlation in some Lower Paleozoic rocks and oilsand hints at a complex early history for pelagophyte algae. On acautionary note though, researchers should be wary of readilyassigning a lacustrine environmental setting for the parent sourcerocks of later Cambrian and Late Ordovician–early Silurian oilsbased on the absence of 24-npc, which is the single most com-monly used biomarker compound used to discern a marine settingacross Neoproterozoic and Phanerozoic time.

4. Conclusions

Although widely identified in most Phanerozoic marine rocksand oils and with an earliest confirmed robust occurrence in theNeoproterozoic (Love et al., 2009; Kelly et al., 2011), 24-n-propyl-cholestane is not always present above detection limits in marinerocks in certain extended intervals of the Lower Paleozoic era usinghighly sensitive and selective MRM–GC–MS methods of detectionfor C30 steranes. Indeed this biomarker seems very rare in occur-rence and abundance (< 0.5% of total C27–C30 steranes) throughan apparently continuous interval spanning at least 9 my and pos-sibly as much as 20 my in Late Ordovician to early Silurian sedi-mentary deposits. Trace or undetectable amounts of C30 steranes(both 24-npc and 24-ipc), along with elevated 3b-methylhopanes(Rohrssen et al., 2013), appear to be characteristic of Late Ordovi-cian to early Silurian marine rocks. Lack of 24-npc sterane signalis not due to lack of overall sterane biomarker signal or poor sig-nal/noise ratios, and even though some of the samples analyzedwere organic lean carbonates, this scarcity of 24-npc was alsodetected in contemporaneous, organic rich mudstones with goodsource rock potential. Source organisms that synthesized 24-npcsteroids, most likely pelagophyte algae, do not appear to haveflourished as major phytoplankton in Late Ordovician–early Silu-rian seas or in the Middle–Late Cambrian interval associated withthe SPICE event. The hiatus in the occurrence of the 24-npc isobserved in a range of paleoceanographic settings and in a varietyof sedimentary rock types with different organo- and lithofacies.Further MRM–GC–MS analyses will help constrain when 24-n-pro-pylcholestane becomes more abundant in the Silurian era.

Acknowledgments

Funding for this project was provided by an Agouron Institutegrant to G.D.L. and through a Dissertation Year Fellowship and anAgouron Geobiology Fellowship awarded to M.R. The authors wishto thank Brian Witzke and the Iowa Geological Survey for access tocores; D. Boulet and the Société des établissements de plein air duQuébec for assistance with work in Anticosti National Park; as wellas D. Jones, D. Fike, S. Finnegan and W. Fisher for assistance in thefield and collecting core at the Geological Survey of Canada’s geo-logical core and sample repository. Finally, the authors acknowl-edge constructive feedback from two anonymous reviewers.

Associate Editor—Courtney Turich

M. Rohrssen et al. / Organic Geochemistry 80 (2015) 1–7 7

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