Tremolada, F., and Bralower, T. J., 2004 - Penn State University

F. Tremolada, T.J. Bralower / Marine Micropaleontology 52 (2004) 107–116 109not represented due to an unconformity (Müller,1979).DSDP Site 213 was drilled on the eastern side ofthe Ninetyeast Ridge in the Indian Ocean at a palaeolatitudeof 28jS (Fig. 1). A sedimentary successionspanning the Upper Paleocene to Holocene was recovered(von der Borch et al., 1974). The PETMinterval lies in Core 16 and consists of brown, clayeysediments and nannofossil oozes of latest Paleocene–earliest Eocene age, directly overlying basement. Thebrown clay contains the upper part of the CIE indicatingthat the onset of the event is missing (Ravizzaet al., 2001).The PETM at both sites was sampled every centimetre.Quantitative analyses were carried out on 120samples from Sites 213 and 78 samples from Site 401.The PETM interval was investigated in 49 samplesfrom Site 213 and 51 samples from Site 401. Smearslides were prepared from well-mixed and unsettledsediment suspensions, and no ultrasonic cleaning orcentrifuge concentration was applied in order to retainthe original nannofossil composition. Nannofossilswere observed in the light microscope at 1250 magnification and identified following the taxonomicschemes of Aubry (1984, 1988, 1989, 1990), Perch-Nielsen (1985), and Varol (1998). Three hundredspecimens were counted in each smear slide. The firstseries of analyses on samples from Site 401 revealedthat the genus Toweius dominated the nannofossilassemblages. Supplementary counts of 250 specimens,excluding the genus Toweius, were thus undertakenin order to raise the percentages of other taxa tostatistically significant levels (Bralower, 2002). Interpretationsat Site 401 are based on the non-Toweiusfraction. If possible, specimens were identified at thespecies level during counts, and grouped at thegeneric level for analysis (Figs. 2–5).3. ResultsThe calcareous nannofossil assemblages throughthe PETM at Sites 401 and 213 show similar trends,but the magnitudes of the fluctuations are different.Nannofossils at Site 401 are very well preserved anddisplay a high species richness throughout the intervalFig. 2. Calcareous nannofossil (placoliths and Octolithus multiplus) assemblage fluctuations across the PETM at Site 401. NP biozonation afterMartini (1971); CP biozonation after Okada and Bukry (1980). Grey band indicates thermal maximum. Dashed line marks the boundarybetween the lower and upper parts of PETM.

110F. Tremolada, T.J. Bralower / Marine Micropaleontology 52 (2004) 107–116Fig. 3. Calcareous nannofossil (holococcoliths and nannoliths) assemblage fluctuations across the PETM at Site 401. NP and CP biozonations asin Fig. 2. Grey band indicates thermal maximum. Dashed line marks the boundary between the lower and upper parts of PETM.investigated, with a slight decrease in species richnessin the lower portion of the PETM (Figs. 2 and 3).Assemblages are dominated by the genus Toweius,which comprises 45–74% of the entire assemblage.Other genera, such as Discoaster, Sphenolithus, Fasciculithus,Ericsonia, and Chiasmolithus, and thespecies Octolithus multiplus, rarely constitute 10%of the total assemblages.Counts from the interval below the PETM showthe typical Upper Paleocene nannofossil assemblagesof northern high latitudes, with high abundances ofToweius spp., Octolithus multiplus and Chiasmolithusspp. However, a significant abundance peak of Biscutum?sp. (Biscutum sp. of Bralower, 2002) and Prinsiusbisulcus was recognized just below the onset ofthe PETM. The PETM interval can be subdivided intoa lower part and an upper part on the basis of thenannofossil assemblages. The lower part is characterizedby a sharp increase of Discoaster, Fasciculithus,and Ericsonia spp. The genus Fasciculithus is mainlyrepresented by Fasciculithus tympaniformis, whereasother species display very low abundances. Discoastermultiradiatus and Discoaster lenticularis rangefrom 8% to 15% of the non-Toweius fraction, andlong-armed and asymmetrical discoaster species, suchas Discoaster araneus, Discoaster anartios, and Discoasterokadai have their first occurrence during thistime interval, as previously documented by Cramer etal. (2000), Monechi et al. (2000), Bralower (2002),and Kahn and Aubry (2004).In the upper part of the PETM, a decrease inabundance of Discoaster spp. and Fasciculithus spp.is evident, whereas Toweius spp. and Zygrhablithusbijugatus abruptly increase in abundance. Sphenolithusspp. increase slightly in abundance in the upperpart of the PETM, and consist largely of Sphenolithusmoriformis, Sphenolithus primus, and Sphenolithusanarrhopus; however, this group accounts for less

112F. Tremolada, T.J. Bralower / Marine Micropaleontology 52 (2004) 107–116than 10% of the nannofossil assemblages in allsamples. The genus Chiasmolithus is either rare orabsent in the PETM interval. The last occurrence ofOctolithus multiplus at Sites 401 and 1051 (BlakeNose, NE Atlantic) has been observed at the onset ofthe PETM (Kulhanek and Watkins, 2002).Counts of samples from Site 213 indicate markedchanges in the composition and preservation of theassemblages through the interval analyzed (Figs. 4 and5). Preservation in samples ranges from moderate togood. The brown claystones display assemblages withvery low species richness ( < 20 species), almost entirelycomposed of solution-resistant taxa, whereasassemblages in the oozes are better preserved andricher (>40 species). Brown claystone samples aredominated by nannoliths, such as Fasciculithus spp.,Sphenolithus spp., and Discoaster spp., and a fewplacolith taxa, such as Ericsonia spp., Campylosphaeradela, and Pedinocyclus praelarvalis (see Bralowerand Mutterlose, 1995). The abundance ofToweius is < 15% of the entire assemblage and Chiasmolithusis absent. The calcareous oozes overlying thebrown claystone show very different assemblage compositions,with sharp drops in the abundances ofFasciculithus spp. and Discoaster spp., and a lesspronounced decrease in the abundance of Sphenolithusspp., P. praelarvalis, and C. dela. The genus Toweiusbecomes very abundant (>20% of the nannofossilassemblages), but never as dominant as at higherlatitudes. In addition, the holococcolith species, Zygrhablithusbijugatus, displays a significant abundanceincrease (up to 10%) and an inverse correlation withthe genus Fasciculithus. The abundances of Chiasmolithusspp. and Biscutum? sp. increase slightly, but are< 5% throughout the section.4. Calcareous nannofossil palaeoecologyCalcareous nannoplankton are sensitive to environmentalchanges in their surface water habitats; thus,changes in the abundance of fossil assemblages can beinterpreted as a response to palaeoceanographic perturbations.Several palaeobiogeographic studies haveaddressed the ecological affinities of a number ofPaleogene taxa (Haq and Lohmann, 1976; Haq etal., 1977; Backman, 1986; Wei and Wise, 1990;Hallock et al., 1991; Aubry, 1992, 1998; Bralower,2002): the genera Discoaster and Ericsonia appear tohave tolerated warm and oligotrophic waters, whereasBiscutum? sp., Chiasmolithus, and Toweius are likelyindicators of colder and more eutrophic waters. Inaddition, Zygrhablithus bijugatus was interpreted asan oligotrophic species by Aubry (1998). The ecologicalaffinities of Fasciculithus and Sphenolithus areunclear, even though these genera seem to display aninverse correlation with Prinsius martinii—a speciesthat is abundant in eutrophic, high-latitude environments(Haq and Lohmann, 1976).The nannofossil assemblage fluctuations observedthrough the PETM at Sites 213 and 401 correlate withthose at southern high-latitude ODP Site 690 (Bralower,2002) and the Tethyan section at Alamedilla,southern Spain (Monechi et al., 2000). Toweiusincreases in abundance in the later part of the PETMat Sites 690 (Bralower, 2002) and 401, but it showslow abundances in the brown claystones of Site 213.This genus is abundant at higher latitudes and hasbeen interpreted as a mesotrophic form (Bralower,2002).The nannofossil assemblages in the lower part ofthe PETM at Sites 401 and 690 are thus indicative ofwarm and oligotrophic conditions, whereas the upperportion of the event at these sites were likely characterizedby more eutrophic conditions (Bralower,2002). The high abundances of Fasciculithus spp.are not easily interpretable, but may indicate oligotrophicconditions.5. InterpretationsThe Paleocene–Eocene transition shows a range ofsignificant biotic and geochemical changes. Bloomsof the dinoflagellate genus, Apectodinium, in coastalenvironments (Fig. 6) are likely indicative of highsurface water productivity and increased temperature(Crouch et al., 2001). Benthic foraminiferal assemblagesat open ocean and shelf sites are thought to beindicators of high nutrient concentrations, likely as aresult of eutrophic conditions (Thomas and Shackleton,1996; Speijer and Schmitz, 1998; Thomas, 1998;Thomas et al., 2000).High biogenic barium accumulation rates at openocean PETM sections have been interpreted as evidencefor widespread high productivity (Bains et al.,

F. Tremolada, T.J. Bralower / Marine Micropaleontology 52 (2004) 107–116 113Fig. 6. Summary of biotic, geochemical, and volcanic events, which occurred across the interval of the PETM. Calcareous nannofossil eventsare a synthesis of the results from Sites 213, 401, and 690.2000). However, the interpretation of barium as aproxy for productivity is problematic due to its shortresidence time (f 8 kyr) and the potential increase inbarium fluxes as a result of methane dissociation(Dickens et al., 2003).Organic carbon content is difficult to interpret interms of productivity, but provides some information.Shallower water sections, in Kazakhstan and Uzbekistan,show highly anoxic conditions and organicmatter of marine origin, with total organic carbon(TOC) ranging from ca. 0.2% to 8% (Bolle et al.,2000). These concentrations are possibly indicative ofhighly productive surface waters.Evidence of dysoxic conditions is recorded infinely laminated sediments at ODP Site 999 in theColombia Basin (Caribbean Sea), where TOC is< 0.3% (Bralower et al., 1997)—only slightly higherthan in the underlying and overlying sediments. Thedecrease in the degree of bioturbation at the beginningof the PETM at Site 690 (Thomas and Bralower,2001; Bralower, 2002) is associated with very lowTOC (f 0.1%). The finely laminated sediments atSite 999 and the decrease of bioturbation at Site 690indicate diminished deep-water oxygenation, but thelack of organic matter at these sites has been interpretedto reflect low organic carbon fluxes fromsurface waters as a consequence of oligotrophy (Bralower,2002).Open ocean nannofossil assemblages at Sites 213,401, and 690 are generally indicative of low productivity,in contrast with more proximal settings, whichwere characterized by higher productivity (Fig. 7), asdocumented by major changes in dinoflagellateassemblages (Crouch et al., 2001).The peaks in Biscutum? sp. and P. bisulcusimmediately below the CIE at Sites 401 and 690,interpreted as an increase in fertility of surfacewaters (Bralower, 2002), might be unrelated to thePETM.The formation of the NAIP likely triggered greenhouseconditions during the Paleocene–Eocene transition(Eldholm and Thomas, 1993), and possiblycaused the release of methane hydrates (Fig. 7). Theoxidation of hydrates consumed a large amount ofoxygen in the ocean (Thomas et al., 2002) and addeda massive amount of carbon dioxide to the oceans andatmosphere (e.g., Dickens et al., 1995, 1997). Inproximal settings, increased runoff (Robert and Kennett,1994) could have led to high productivity, which,combined with low oxygen concentrations, resulted inhigh TOC contents in the sediments. Lower TOCcontents in open ocean sites can be explained by a

114F. Tremolada, T.J. Bralower / Marine Micropaleontology 52 (2004) 107–116Fig. 7. Palaeoceanographic model for causes and effects of the PETM in greenhouse conditions.combination of somewhat higher oxygenation andlower productivity (Fig. 6).The PETM triggered the rapid radiation of theplanktonic foraminifera (e.g., Kelly et al., 1996) andland mammals (e.g., Clyde and Gingerich, 1998).The rate of extinction in the benthic foraminiferalcommunities varies from tropical to high latitude sites(e.g., Kaiho et al., 1996; Speijer et al., 1996). Theextinction of the northern temperate-latitude and highlatitudenannofossil species, Octolithus multiplus, waslikely caused by increasing temperature, and theelimination of Fasciculithus possibly resulted fromcompetition with Zygrhablithus bijugatus. Furthermore,palynofloras show no extinctions and onlymoderate changes in composition and diversity duringthe Paleocene–Eocene transition (e.g., Wing andHarrington, 2001).6. ConclusionsNannofloral changes associated with the PETMtransition at DSDP Sites 213 and 401 correlate withthose documented from several other locations, suchas ODP Site 690 (Bralower, 2002) and Alamedilla,southern Spain (Monechi et al., 2000). The PETM canbe subdivided into a lower part and a upper part on thebasis of nannofossil assemblages. The lower part ischaracterized by high abundances of Discoaster spp.,Ericsonia spp., and Fasciculithus spp., interpreted asindicators of warm surface waters and probably oligotrophicconditions (Wei and Wise, 1990; Aubry,1992; Kelly et al., 1996; Bralower, 2002). The upperpart shows an increase in abundance of Toweius spp.and Zygrhablithus bijugatus, which likely reflectsmore eutrophic and cooler conditions.The major nannofloral changes at DSDP Site 213seem to be strongly affected by diagenesis. Theincrease in abundance of genera most resistant todissolution, such as Discoaster, Ericsonia, and Fasciculithus,inversely correlates with the nannofossilspecies richness at Site 213.The appearance of asymmetrical Discoaster andRhomboaster species might indicate the developmentand availability of new ecological niches, resultingfrom major palaeoceanographic changes during thePETM (Bralower, 2002). The last occurrence ofOctolithus multiplus might be a reliable bioevent atnorthern intermediate and high latitudes.AcknowledgementsWe gratefully acknowledge K. von Salis, J. Young,J. Lees, and an anonymous reviewer for criticalcomments that helped us improve the manuscript.Many thanks to I. Premoli Silva, E. Erba, I. Raffi, J.Backman, and G. Dickens for thoughtful discussions.Samples were provided by the Ocean DrillingProgram, and we thank the curatorial staff for their

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