Role of Pro-Thrombolites in the Geomorphology of a Coastal … · 2014. 5. 16. · thrombolite...

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Role of Pro-Thrombolites in the Geomorphology of a Coastal Lagoon1

D. A. Siqueiros-Beltrones2

Abstract: Thrombolites are lithified biosedimentary structures generated byentrapment, precipitation, and binding of sediments promoted by growth andmetabolic activity of cyanobacteria. Beaches of the coastal lagoon known asEnsenada de La Paz in Baja California Sur, Mexico, are bordered by sedimen-tary formations of cyanobacterial origin identified as pro-thrombolites (incip-ient thrombolites) that represent a first record for the region and Mexico.Observed thrombolithic structures show grains of varied sizes embedded withina fine-grain micritic matrix, which may be surrounded by medium-grain cement-ing micrite. Different degrees of consolidation occur—some crumble easily,whereas others require some manual force to break. These pro-thrombolitesconsist of platforms >20 cm thick and/or fragments of assorted sizes and forms.In some cases the structures have lithified, forming rocky plates (thrombolites).The extension and wide distribution of pro-thrombolites around the La Paz la-goon suggests that these structures could have determined its evolution froman original (primitive) cove into a lagoon. That is, the formation of pro-thrombolites through the entrapment and binding of sediments may have even-tually altered water circulation, promoting sand sedimentation causing the for-mation of the El Mogote sand bar. Likewise, pro-thrombolites may have formedlarge extensions of headlands through accretion. Thus, several square kilometersof populated land around the La Paz lagoon may have thrombolithic origin.

Microbial structures seem to be a com-mon feature along the coasts of the Baja Cal-ifornia Peninsula. Ephemeral microbial matshave been seen to form in some places whereconditions change abruptly (Siqueiros-Beltrones 1988, 1990). However, a large partof the beaches of the Ensenada de La Paz(Baja California Sur, Mexico) coastal lagoonare covered by permanent microbial matsand bordered by biosedimentary structures

generated or promoted by the growth of fila-mentous cyanobacteria. These structureswere tentatively identified as living or in-cipient thrombolites during a samplingfor benthic diatoms in assorted substrata(Siqueiros-Beltrones 2006). Thrombolitesare biosedimentary structures generated bythe entrapment, binding, and consolidationof sediments as a result of the metabolicactivity and growth of microorganisms, par-ticularly cyanobacteria. Contrary to the struc-ture of stromatolites, in thrombolites nolamination is observed (Charpy et al. 1999,Riding 2000, Shapiro 2000, Stal 2000). Atleast five types of thrombolites may be recog-nized, all characterized by their clotted struc-ture (Riding 2000).

In this study the term ‘‘pro-thrombolite’’ ispreferred over ‘‘living thrombolites’’ for thestructures found in the La Paz lagoon thatshow no lithification. These, unlike the con-spicuous microbial mats associated withthem, have accreted by trapping sediments,forming a clotted, semiconsolidated biosedi-mentary structure.

Pacific Science (2008), vol. 62, no. 2:257–269: 2008 by University of Hawai‘i PressAll rights reserved

1 This study was supported by grants from InstitutoPolitecnico Nacional through projects CGPI-20040024and 20050069. The author is a Comision de Cooperaciony Fomento de Actividades Academicas and Estımulos alDesempeno de los Investigadores fellow (Instituto Poli-tecnico Nacional). Manuscript accepted 26 June 2007.

2 Departamento Plancton y Ecologıa Marina, CentroInterdisciplinario de Ciencias Marinas (CICIMAR), In-stituto Politecnico Nacional, P.O. Box 592, La Paz,B.C.S., Mexico, 23000 (e-mail: [email protected]).

Fossil thrombolites have been observedfarther north in the Baja California Peninsulain the area of Bahıa Concepcion (Miranda-Avilez et al. 2005), where living hot-spring(laminated) stromatolites are also found(Canet et al. 2005). However, pro-thrombolites have not been hitherto reportedin any Mexican marine environment. Al-though pro-thrombolites were apparentlynoted at El Centenario in the Ensenada deLa Paz (Figure 1) by Cruz-Orozco et al.(1989), they were recorded as beach rock.Beach rock formation has been related tostromatolites, and these require environmen-tal conditions similar to those thrombolitesneed to develop (Charpy et al. 1999). In fact,some controversy exists over whether the ori-gin of beach rock is biotic (cyanobacteria) orabiotic, though both factors seem to contrib-ute (Krumbein 1979). In a recent review ofthe geology of the La Paz lagoon no otheraccount of beach rock is mentioned (Alvarez-Arellano et al. 1997).

Cyanobacterial structures distributed alongthe Baja California Peninsula have beenstudied to understand the processes by whichstromatolites are formed (Horodysky and vonder Haar 1975, Horodysky 1977, Marguliset al. 1980) and to provide clues of past mi-crobial activity. But do these biosedimentarystructures play other roles of ecological orgeological importance? Many questions re-main unanswered: What is the proportionaltaxonomic composition of cyanobacteria invarious stages of the formation of thrombo-lithic structures? What is the growth rateof the said structures? How are sedimentstrapped and what determines their texture?How does lithification occur and what is theprecise role of cyanobacterial mats in thisprocess? How do these structures affect man-grove settlement and do they interact topromote progradation of the lagoon mar-gins? How extensive are pro-thrombolithic–generated grounds?

Here I address only the first question. My

Figure 1. Location of sites where pro-thrombolites were first surveyed in the Baja California Peninsula, and siteswhere lithified thrombolites were found (Estero Zacatecas, El Conchalito [to the east of CICIMAR], and ElMogote).

258 PACIFIC SCIENCE . April 2008

initial observations led to the assumption thatpro-thrombolithic activity might have had animportant role in the geomorphologic originof the La Paz lagoon and headlands. Thisprompted the hypothesis that seaward pro-thrombolite accretion influenced the trans-formation of the primitive cove (the En-senada de La Paz) into a lagoon by providingthe necessary spit (Nava-Sanchez and Cruz-Orozco 1989) for the accumulation of sand.This eventually led to the formation of theEl Mogote sand bar (Figure 1). Moreover, inother parts of the lagoon pro-thrombolithicgrowth may have directly generated large ex-tensions of land through progradation. Tosupport this hypothesis the following sub-ordinate hypotheses were contrasted: (1) Be-cause living thrombolites contribute to theprogradation of the lagoon margins due totheir seaward growth, active pro-thrombolites(exposed even at high tide) should occur overmuch of the lagoon margins; (2) Thrombo-lithic structures are to be found extendinglandward, either buried or exposed; (3) Lithi-fied thrombolithic structures (thrombolites)should be found landward (perhaps as beachrock) behind the mangrove fringe; (4) Con-solidated thrombolithic structures, includinglithified forms, should be present at the baseof El Mogote and at the base of other sea-ward land projections inside the lagoon.

Study Area

The Ensenada de La Paz coastal lagoon is lo-cated within Bahıa de La Paz, between 20�

06 0 and 24� 11 0 N and 110� 18 0 and 110� 26 0

W, and separated from it by the El Mogotesand bar (Figure 1). The tidal regime is semi-diurnal and the flow from high to low tide ismore intense, causing strong currents thatkeep the lagoon from filling up by sedimentaccumulation. Salinities vary between 34 and36 ppt, depending on the season ( Jimenez-Illescas et al. 1997). Based on temperaturevariation, three periods may be distinguished:(1) March–June, (2) July–October, and (3)November–February; the last two are consid-ered rainy seasons (Salinas et al. 1990). At themouth of the lagoon, mean temperatures of

23.5�C in April–June and 30.25�C in July–September have been recorded (Cervantes-Duarte et al. 2001).

materials and methods

Initially, samplings were made at four local-ities inside Ensenada de La Paz: (1) ElConchalito, on the campus of Centro deInterdisciplinario de Ciencias Marinas(CICIMAR)–Instituto Politecnico Nacional(24� 08 0 29 00 N and 110� 21 0 07 00 W ); (2) thebeach of El Centenario (24� 06 0 38 00 N and110� 25 0 04 00 W ); (3) northwest of the Centrode Investigaciones Biologicas del Noroeste(CIBNor) (24� 08 0 35 00 N and 110� 25 0 47 00

W ); and (4) Los Bajos (24� 09 0 01 00 N and110� 25 0 49 00 W ), named after the con-spicuous mudflats during low tide (Figure 1).At all localities, fragments from thrombolithicformations were collected using a hammerand spatula. Approximate measurementswere made of the thrombolithic beds. Salinityof water and sediments, and water tempera-ture were measured.

To test the four hypotheses, further explo-rations were conducted: searching for buriedthrombolithic structures behind the man-grove line (hypotheses 1 and 2) and lookingfor different degrees of consolidation ofthrombolithic structures up to lithified forms(hypotheses 3 and 4). These were carried outmainly at the base of El Mogote and at ElConchalito (Figure 1), where thrombolithicstructures were located visually (exposedplates) or by poking with a shovel along themudflats at low tide. Later, a final explorationwas conducted at a beach in El Mogote di-rectly across from CICIMAR to check an-other report of beach rock, which wasassumed to be thrombolithic in nature. Pho-tographs were taken of the different types offormations.

In the laboratory, filamentous cyanobac-teria were scraped off and separated using adissecting needle for observation in fresh(squash) preparations. Identification followedthe works by Charpy and Larkum (1999),Lopez-Cortes (1999), and Whitton and Potts(2000). A part of each scraping was processed

Pro-Thrombolites in a Coastal Lagoon . Siqueiros-Beltrones 259

for the mounting of diatoms, which were in-vestigated separately (Siqueiros-Beltrones2006). The thrombolite fragments were ex-amined to determine their degree of consoli-dation and to make thin sections in thegeology laboratory.

results

Description of Pro-Thrombolites

The thrombolithic structures in Ensenada deLa Paz lagoon consist of conglomerated orclotted sediments of different textures. Thedegree of consolidation varies from semicon-solidated (i.e., those that crumbled more orless easily [pro-thrombolites]) to hard lithi-fied plates (thrombolites). Frequently, wholeor fragments of clam shells were seen em-

bedded in the pro-thrombolites, as well asfouling by balanoids. The overall area of theformations has not been estimated. They in-clude extensive platforms and fragments ofassorted sizes and forms that together amountto varied areas resembling sandstone or (fos-sil) muddy sandstone. The first recognizedpro-thrombolite is located to the north ofCICIMAR (Figure 2). The exposed spitcovers approximately 85 m2 and is seasonallyoccupied by Salicornia sp. On other pro-thrombolithic formations in the vicinity,mangrove trees are anchored (Figure 3).

At El Centenario discontinuous pro-thrombolithic platforms of approximately 40and 30 m2 can be observed. However, theCIBNor locality showed the best-definedplatforms, consisting mainly of consolidatedsediments with a discontinuous covering of

Figure 2. First thrombolithic formation observed, at El Conchalito (CICIMAR).


cyanobacteria (Figure 4). The live portion ex-tended almost 340 m2, but other platformscould be seen extending to the southeastof CIBNor. Finally, at Los Bajos pro-thrombolites extended about 60 m2, althoughother formations were distributed to thenorthwest.

Water temperature varied between 24 and29�C, and salinity from 35 to 40 ppt, whichare in the previously reported ranges for thearea. Sediments showed a maximum variationof 34 to 100 ppt at El Centenario in October.

Associated Taxa

Active pro-thrombolites are covered by acomplex sheet of microalgae, principally cya-nobacteria and diatoms. The main cyanobac-teria taxa were Microcoleus chthonoplastes, whose

long multiseriate filaments (Figure 5a,b,c) ex-hibit thick sheaths, and Oscillatoria limosa/Lyngbya aestuarii (Figure 5d). Also commonwere thinner unidentified oscillatoriales thatbreak down into hormogonia, plus Spirulinasp., Oscillatoria spp., Calothrix, and unicellularforms Chroococcus turgidus and Aphanotece sp.

Extensive cyanobacterial mats, closely as-sociated with the pro-thrombolites, are seendistributed homogeneously all over the ex-posed mudflats in the lagoon during low tidesand extending subtidally. Abundant snails(Cerithidia mazatlanica) may be seen grazingactively on and around the formations.

The diatom survey (Siqueiros-Beltrones2006) yielded 150 taxa, mostly pennate raphidforms, although araphid and centric formswere frequent. Observation of fresh samplesshowed that diatoms were alive, indicating

Figure 3. Pro-thrombolite bed showing an anchored mangrove (Avicennia germinans) at El Conchalito (CICIMAR).


that an assemblage actually develops on thepro-thrombolites. The high species richnessdoes not differ from that of other assem-blages in the region, and many taxa indicatethe influence of the local mangrove system(e.g., 10 species of Diploneis, six species ofLyrella, and eight species of Mastogloia),whereas several others are commonly associ-ated with cyanobacterial mats (e.g., Gyrosigmascalproides var. eximium and Mastogloia pumila).However, their abundance was much lowerthan in the surrounding sediments. More-over, dense mats of pennate diatoms were ob-served surrounding pro-thrombolites (LosBajos), with a different species composition.

During a second exploration, other pro-thrombolites were observed to the east of CI-CIMAR and were common up to the base ofEl Conchalito Peninsula (Figure 6), where

supralitoral formations occurred. Further-more, buried compact structures >20 cmthick were located by digging at the easternCICIMAR campus limits. Together theseconstitute evidence of extensive ground for-mation. Furthermore, at El Comitan (thebeach where CIBNor is located) and El Cen-tenario pro-thrombolithic structures are com-mon in the upper intertidal, and in bothlocalities large areas of ground appear tohave been generated (Figure 7a,b). In the lat-ter, extensive cyanobacterial mats can be ob-served in the exposed intertidal showingraised surfaces due to the accumulation un-derneath of very fine sediments. These wereinterpreted as representing incipient pro-thrombolites (Figure 8a,b).

In the area known as Estero Zacatecas,where El Mogote forms an angle with the

Figure 4. Extensive rocklike pro-thrombolithic platforms at CIBNor during low tide.


coastline (Figure 1), pro-thrombolites werecommon either as isolated structures (Figure9) or extending behind the mangrove fringeand serving as substrate for Salicornia sp.plants.

Thrombolites

On higher mudflats of Estero Zacatecas,lithified plates (thrombolites sensu stricto)were discovered (Figure 10). This agreeswith the hypothesis that both pro-thrombolites and thrombolites (lithifiedplates) would be found at the base of the ElMogote sand bar. Thus, under the assump-tion that this should also hold for other pointsthat project seaward such as El Comitan andEl Conchalito, I explored landward behindthe mangroves at both localities. There I

found structures showing different degreesof consolidation (from hand breakable tostonelike), including fragmented plates. Thelithified plates were smaller than the onesfrom Estero Zacatecas and varied from 1 to2 cm thick with compacted sediments ad-hered underneath.

The first series of petrographic analyses ofthrombolites showed that lithification ofclotted sediments is accomplished throughcementation by micrite. Microscopic exami-nation of thin sections under 10� magnifica-tion showed grains of various sizes includingquartz, feldspar, olivine, phosphorite, oolites,shell fragments, and amphibolites. Mostgrains were rounded, but subrounded and an-gular grains were also common. These char-acteristics and grain size varied in samplesfrom different sites. Grains were embedded

Figure 5. Microcoleus chtonoplastes: a, Multiseriate filament with thick mucilaginous sheath (1,000�); b, apex of tri-chomes inside the sheath (1,000�); c, filament at 200�. Oscillatoria limosa/Lyngbya aestuarii, d.


Figure 6. View of El Conchalito Peninsula and original sampling site (P), new sites with pro-thrombolites (P2), andthick semiconsolidated (larger white oval) and lithified plates (smaller white oval). Mangrove swamp, M. Modifiedfrom Google Earth (Europa Technologies 2007).

Figure 7. Intertidal pro-thrombolithic formations in (a) El Centenario and (b) El Comitan showing erosion by waveaction.

Figure 8. Panoramic (a) and close-up (b) views of accreting cyanobacterial mats that may represent incipient pro-thrombolites at El Centenario, Ensenada de La Paz lagoon, B.C.S.

Figure 9. Exposed pro-thrombolite behind the mangrove line at Estero Zacatecas.

within a fine-grain micritic (microcrystallinecalcite) matrix and were sometimes sur-rounded by medium-grain cementing micrite,which was also observed filling pores. Poros-ity varied from 10 to 40%. No lamination ex-isted in these structures, and no traces ofcyanobacteria or diatoms were detected.

At an El Mogote beach across the channelnorth of CICIMAR, pro-thrombolites serveas anchoring points for mangrove trees.In one site, various stages of thrombolithicformations are present, including cyanobacte-rial mats, pro-thrombolites with various de-grees of consolidation, and thick, lithifiedthrombolithic plates just above the intertidallevel. Thick thrombolites found nearbyshow more complex development (Figure11), and others exhibit what appear to be rhi-zoliths projecting from beneath the lithifiedplates.

discussion

The term ‘‘thrombolite’’ is derived from thefact that trapped sediments have been con-glomerated and consolidated by cyanobac-teria and become lithified without alaminated structure. In many cases, after thetrapping and consolidation of the sedimentshas occurred, they remain active and growing,hence defining the pro-thrombolite stage.Eventually they may become inactive, al-though without reaching lithification, thusforming sedimentary (pro-thrombolithic)platforms, as in El Centenario and El Comi-tan.

The thrombolites found in Ensenada deLa Paz lagoon are different from the ones inAustralian hypersaline lakes but resemble livestromatolites from the Bahamas and Australia(Whitton and Potts 2000). Closer compari-

Figure 10. One of many thrombolithic plates found in Estero Zacatecas mudflats.


son is needed to determine whether all ofthese pertain to the same type of thrombo-lithic structures (Riding 2000) or if a differenttype forms in the Ensenada de La Paz lagoonunder certain conditions.

Most information on thrombolites derivesfrom the study of fossil deposits. Thrombo-lites became conspicuous in the Proterozoic-Cambrian boundary (Riding 2000). In BahıaConcepcion, B.C.S., fossil deposits includerhodolith beds, muddy sandstone, rhizoliths,and mangroves ( Johnson and Ledesma-Vasquez 1997) as well as stromatolites andthrombolites (Miranda-Avilez et al. 2005).Recent and fossil stromatolites are also associ-ated with the presence of hot springs (Canetet al. 2005).

The formations described in Ensenada de

La Paz represent a first record of thrombo-lites from a nonfossil deposit for the regionand Mexico, which are quite recent, based onthe estimated age of the lagoon (5,000–6,000yr [Nava-Sanchez and Cruz-Orozco 1989]).Likewise, this is the first time that the identi-fication of pro-thrombolites as active growingpredecessors to thrombolites and their role inthe geomorphology of coastal lagoons are ad-dressed. In this perspective, environmentalconditions that determine thrombolite forma-tion are actually playing a larger role in thetransformation of ecosystems.

Shoaling conditions, frequent exposure,and low dynamics in the lagoon seem to beplaying an important part in the formationof thrombolithic structures. These, togetherwith high concentrations of carbonates re-

Figure 11. Thick thrombolithic formations at an El Mogote beach showing complex growth form.


quired for their formation (Riding 2000), en-hance the thrombolithic processes in the En-senada de La Paz lagoon. Thus, althoughenvironmental conditions necessary for theformation of thrombolites are often consid-ered extreme (Charpy et al. 1999), in the En-senada de La Paz this does not seem to be thecase because water temperature and salinityvariation occurs within the reported rangesfor the area. Although sediments may reachextreme salinities (100 ppt), the density andspecies richness of pennate diatoms observedin the sediments surrounding the pro-thrombolites indicate that conditions actuallyfavor microalgal growth. Thus, currentlythere is no clear explanation for the develop-ment of cyanobacteria mats and the eventualformation of pro-thrombolites.

In conclusion, besides providing clues forpast microbial activity, cyanobacterial matsgenerate pro-thrombolithic structures, whichin turn play important ecological or geolog-ical roles (i.e., hitherto overlooked land-forming coastal processes). My observationssupport the proposed hypothesis that pro-thrombolithic development triggered thebirth of the El Mogote sand bar by providingthe necessary spit for the accumulation ofsand and, through continuous growth for ap-proximately six millennia, eventually trans-formed a primitive cove into the moderncoastal lagoon known as Ensenada de LaPaz.

In other parts of the lagoon margins,mainly at El Centenario, El Comitan, and ElConchalito Peninsula, where CICIMAR andthe Centro de Estudios Tecnologicos delMar (CETMAR) high school campuses stand,extensive pro-thrombolithic development isevidenced by exposed and buried supralittoralformations. These strongly indicate that theseformations have generated large extensions ofland (progradation) currently occupied byurban settlements of La Paz city. In El Com-itan and El Centenario a different strategy inthe search for lithified structures may be re-quired due to the greater expansion of humansettlements.

Further research is being planned to relatesite conditions to differential thrombolite andpro-thrombolite formation in the lagoon.

The recently located formations found in ElMogote across the channel north of CICI-MAR offer many opportunities for study. Itis clear that much research needs to be done,and this will require preservation of thesemillenary formations. It is hoped that this re-port will draw attention to the value of theEnsenada de La Paz in furthering our under-standing of biosedimentary structures andserve to support any preservation efforts tocome.

acknowledgments

U. Argumedo and O. U. Hernandez-Almeidahelped during the field trips and in sampleand image processing. J. Murillo de Nava ofCICIMAR gave advice on field and labora-tory geology, made the thin sections, andprovided the preliminary petrographic de-scription. I thank R. Shapiro, M. Johnson,and an anonymous reviewer for their preciseobservations.

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