Showing papers by "Ellen Thomas published in 2007"

Chapter Seven Paleoceanographical Proxies Based on Deep-Sea Benthic Foraminiferal Assemblage Characteristics

Abstract: Publisher Summary This chapter focuses on the paleoceanographical proxies based on deep-sea benthic foraminiferal assemblage characteristics, and presents the following three proxy relationships that are promising: those between benthic foraminiferal faunas and benthic ecosystem oxygenation, export productivity, and deep-sea water mass characteristics. Under most circumstances the composition of deep-sea benthic foraminiferal assemblages is controlled by a rather limited number of environmental factors. The available proxies based on benthic foraminiferal assemblage composition show that they have major potential, but further research is needed to add or improve the quantitative aspects. In many cases, such as bottom water oxygenation, and Corg flux to the ocean floor, it can be done by significantly increasing the size of existing databases. In other cases such as periodicity of the organic flux, time series observations are necessary. A major obstacle is insufficient knowledge of the differences between recent and fossil faunas due to taphonomical alterations. This phenomenon, of importance for all paleoceanographic proxies, can to some extent be solved relatively easily in the case of foraminiferal assemblages by detailed studies of their vertical succession in sediments deposited in the last 5,000 years, when environmental conditions were probably rather invariable in many areas. Proxies based on foraminiferal assemblage composition are fundamentally different from all geochemical proxies, and thus may provide independent reconstructions of essential oceanographic parameters.

Cenozoic mass extinctions in the deep sea: What perturbs the largest habitat on Earth?

Abstract: Deep-sea benthic foraminifera live in the largest habitat on Earth, constitute an important part of its benthic biomass, and form diverse assemblages with common cosmopolitan species. Modern deep-sea benthic foraminiferal assemblages are strongly infl uenced by events affecting their main food source, phytoplankton (a relationship known as bentho-pelagic coupling). Surprisingly, benthic foraminifera did not suffer signifi cant extinction at the end of the Cretaceous, when phytoplankton communities underwent severe extinction. Possibly, bentho-pelagic coupling was less strong than today in the warm oceans of the Cretaceous‐Paleogene, because of differences in the process of food transfer from surface to bottom, or because more food was produced chemosynthetically on the seafl oor. Alternatively, after the end-Cretaceous extinction the food supply from the photic zone recovered in less time than previously thought. In contrast, deep-sea benthic foraminifera did undergo severe extinction (30%‐50% of species) at the end of the Paleocene, when planktic organisms show rapid evolutionary turnover, but no major extinction. Causes of this benthic extinction are not clear: net extinction rates were similar globally, but there is no independent evidence for global anoxia or dysoxia, nor of globally consistent increase or decrease in productivity or carbonate dissolution. The extinction might be linked to a global feature of the end-Paleocene environmental change, i.e., rapid global warming. Cenozoic deep-sea benthic faunas show gradual faunal turnover during periods of pronounced cooling and increase in polar ice volume: the late Eocene‐early Oligocene, the middle Miocene, and the middle Pleistocene. During the latter turnover, taxa that decreased in abundance during the earlier two turnovers became extinct, possibly because of increased oxygenation of the oceans, or because of increased seasonality in food delivery. The Eocene-Oligocene was the most extensive of these turnovers, and benthopelagic coupling may have become established at that time.

The Palaeocene-Eocene Thermal Maximum Super Greenhouse: Biotic and Geochemical Signatures, age Models and Mechanisms of Climate Change

Abstract: The Palaeocene–Eocene Thermal Maximum (PETM), a geologically brief episode of global warming associated with the Palaeocene–Eocene boundary, has been studied extensively since its discovery in 1991. The PETM is characterized by a globally quasi-uniform 5–8 8C warming and large changes in ocean chemistry and biotic response. The warming is associated with a negative carbon isotope excursion (CIE), reflecting geologically rapid input of large amounts of isotopically light CO2 and/or CH4 into the exogenic (ocean–atmosphere) carbon pool. The biotic response on land and in the oceans was heterogeneous in nature and severity, including radiations, extinctions and migrations. Recently, several events that appear similar to the PETM in nature, but of smaller magnitude, were identified to have occurred in the late Palaeocene through early Eocene, with their timing possibly modulated by orbital forcing. Although debate continues on the carbon source, the mechanisms that caused the input, the mechanisms of carbon sequestration, and the duration and pacing of the event, the research carried out over the last 15 years has provided new constraints and spawned new research directions that will lead to improved understanding of PETM carbon cycle and climate change. A distinct period of extreme global warmth was initiated close to the boundary between the Palaeocene and Eocene epochs, approximately 55.5 Ma ago (Gradstein et al. 2004). This event, termed the Palaeocene–Eocene Thermal Maximum (PETM), occurred during a time of generally warm, ‘greenhouse’ climate conditions, but stands out against the background warmth as an abrupt and short-lived spike in global temperatures. Evidence for global warming is preserved by the TEX86 0 temperature proxy (Sluijs et al. 2006; Zachos et al. 2006), oxygen isotope (dO) excursions in marine foraminiferal calcite (Fig. 1) (Kennett & Stott 1991; Thomas et al. 2002) and terrestrial carbonates (Koch et al. 1995), increased Mg/Ca values in planktonic and benthic foraminifera (Zachos et al. 2003; Tripati & Elderfield 2005), poleward migrations of (sub)tropical marine plankton (Kelly et al. 1996; Crouch et al. 2001) and terrestrial plant species (Wing et al. 2005), and mammal migrations across high northern latitudes (Bowen et al. 2002, 2006; Smith et al. 2006). Associated with the warming is a negative 2.5–6‰ carbon isotope (dC) excursion (CIE) (Kennett & Stott 1991; Koch et al. 1992; Thomas et al. 2002; Pagani et al. 2006), generally accepted to reflect the geologically rapid injection of C-depleted carbon, in the form of CO2 and/or CH4, into the global exogenic carbon pool (Fig. 1). The apparent conjunction between carbon input and warming has fuelled the hypothesis that increased greenhouse gas concentrations resulted in greenhouse warming during the PETM. The total amount of carbon input during the PETM, which is known to within an order of magnitude (Dickens et al. 1997; Zachos et al. 2005; Pagani et al. 2006), was about 4–8 times the anthropogenic carbon release from the start of the industrial era up From: WILLIAMS, M., HAYWOOD, A. M., GREGORY, F. J. & SCHMIDT, D. N. (eds)Deep-Time Perspectives on Climate Change: Marrying the Signal from Computer Models and Biological Proxies. The Micropalaeontological Society, Special Publications. The Geological Society, London, 323–349. 1747-602X/07/$15.00 # The Micropalaeontological Society 2007. to today (Marland et al. 2005), and comparable to that expected from gross anthropogenic emissions through the end of the 21st century (Intergovernmental Panel on Climate Change 2001). In association with carbon cycle and climatic change, the PETM also stands out as a time of major biotic restructuring. Given the probable ties between releases of near-modern levels of carbon-based greenhouse gases and PETM climatic and biotic change, the PETM has developed as a provocative geological case study in global change, and many of the event’s characteristics and mechanisms are under intensive study. A large volume of research on the PETM has appeared over the past decade (Fig. 2), and in this paper we aim to review and synthesize this material, including the duration and magnitude of carbon cycle perturbation, magnitude of warming, changes in ocean chemistry and marine and terrestrial biotic response. 167

Barite accumulation, ocean productivity, and Sr/Ba in barite across the Paleocene–Eocene Thermal Maximum

Abstract: The Paleocene–Eocene Thermal Maximum (PETM), ca. 55 Ma, was a period of extreme global warming caused by rapid emission of greenhouse gases. It is unknown what ended this episode of greenhouse warming, but high oceanic export productivity over thousands of years (as indicated by high accumulation rates of barium, Ba) may have been a factor in ending this warm period by carbon sequestration. However, Ba has a short oceanic residence time (~10 k.y.), so a prolonged global increase in Ba accumulation rates requires an increase in input of Ba to the ocean, increasing barite saturation. We use a novel proxy for barite saturation (Sr/Ba in marine barite) to demonstrate that the seawater saturation state with respect to barite did not change across the PETM. The observations of increased barite burial, no change in saturation, and the short residence time can be reconciled if Ba burial decreased at continental margin and shelf sites due to widespread occurrence of suboxic conditions, leading to Ba release into the water column, combined with increased biological export production at some pelagic sites, resulting in Ba sink reorganization.

Reappraisal of early Paleogene CCD curves: foraminiferal assemblages and stable carbon isotopes across the carbonate facies of Perth Abyssal Plain

Abstract: Bulk carbonate content, planktic and benthic foraminiferal assemblages, stable isotope compositions of bulk carbonate and Nuttallides truempyi (benthic foraminifera), and non-carbonate mineralogy were examined across ∼30 m of carbonate-rich Paleogene sediment at Deep Sea Drilling Project (DSDP) Site 259, on Perth Abyssal Plain off Western Australia. Carbonate content, mostly reflecting nannofossil abundance, ranges from 3 to 80% and generally exceeds 50% between 35 and 57 mbsf. A clay-rich horizon with a carbonate content of about 37% occurs between 55.17 and 55.37 mbsf. The carbonate-rich interval spans planktic foraminiferal zones P4c to P6b (∼57–52 Ma), with the clay-rich horizon near the base of our Zone P5 (upper)—P6b. Throughout the studied interval, benthic species dominate foraminiferal assemblages, with scarce planktic foraminifera usually of poor preservation and limited species diversity. A prominent Benthic Foraminiferal Extinction Event (BFEE) occurs across the clay-rich horizon, with an influx of large Acarinina immediately above. The δ13C records of bulk carbonate and N. truempyi exhibit trends similar to those observed in upper Paleocene–lower Eocene (∼57–52 Ma) sediment from other locations. Two successive decreases in bulk carbonate and N. truempyi δ13C of 0.5 and 1.0‰ characterize the interval at and immediately above the BFEE. Despite major changes in carbonate content, foraminiferal assemblages and carbon isotopes, the mineralogy of the non-carbonate fraction consistently comprises expanding clay, heulandite (zeolite), quartz, feldspar (sodic or calcic), minor mica, and pyrolusite (MnO2). The uniformity of this mineral assemblage suggests that Site 259 received similar non-carbonate sediment before, during and after pelagic carbonate deposition. The carbonate plug at Site 259 probably represents a drop in the CCD from ∼57 to 52–51 Ma, as also recognized at other locations.

Leg 208 Synthesis: Cenozoic Climate Cycles and Excursions.

Abstract: During Ocean Drilling Program Leg 208, six sites were drilled at water depths between 2500 and 4770 m to recover Cenozoic sediments on the northeastern flank of Walvis Ridge. Previous drilling in this region (Deep Sea Drilling Project [DSDP] Leg 74) recovered pelagic oozes and chalk spanning the Cretaceous/Paleogene (K/Pg), Paleocene/Eocene, and Eocene/Oligocene boundaries. The composite sections, recovered via double and triple coring, provide a detailed history of paleoceanographic variation associated with several prominent episodes of early Cenozoic climate change, including the K/Pg boundary, Paleocene/ Eocene Thermal Maximum (PETM), early Eocene Climatic Optimum, and early Oligocene Glacial Maximum. The PETM interval, the main target of Leg 208, was recovered at five sites along a depth transect of 2.2 km. A prominent red clay layer marks the boundary sequence at all sites. Additionally, two as-yet undocumented early Eocene hyperthermal events were recovered: Elmo and X, dated at ~53.5 and ~52 Ma, respectively. A number of postcruise investigations were undertaken on these critical intervals, principally to improve stratigraphic control and the resolution of proxy records of climate and ocean chemistry, and to better understand the regional impacts of these events on biota. The major contributions of Leg 208 include (1) development of new orbitally tuned chronologies for the Paleocene and lower Eocene, (2) high-resolution characterization of Paleocene/Eocene boundary carbonate dissolution horizons and correlation to the carbon isotope excursion and PETM, (3) development of the first marine-based carbon isotope record of terrestrial n-alkanes for the PETM, (4) documentation of the ecological impacts of the PETM on calcareous algae, (5) resolving the full magnitude of the 1Kroon, D., Zachos, J.C., and Leg 208 Scientific Party, 2007. Leg 208 synthesis: Cenozoic climate cycles and excursions. In Kroon, D., Zachos, J.C., and Richter, C. (Eds.), Proc. ODP, Sci. Results, 208: College Station, TX (Ocean Drilling Program), 1–55. doi:10.2973/odp.proc.sr.208.201.2007 2Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, HV 1081 Amsterdam, The Netherlands. 3Earth and Planetary Sciences Department, University of California, Santa Cruz, Santa Cruz CA 95064, USA. jzachos@es.ucsc.edu 4Scientific Party addresses. Initial receipt: 1 September 2006 Acceptance: 28 February 2007 Web publication: 16 April 2007 Ms 208SR-201 D. KROON ET AL. LEG 208 SYNTHESIS: CENOZOIC CLIMATE CYCLES AND EXCURSIONS 2 carbonate compensation depth shift as well as its timing relative to the onset of Antarctic glaciation in the earliest Oligocene, (6) coupling the middle Miocene high abundances of biserial planktonic foraminifers to changes in regional ocean circulation, (7) constraining the timing of initiation and intensification of North Atlantic Deep Water formation in the Oligocene, (8) increasing the resolution of the Li isotope record for the Neogene, and (9) increasing the resolution of the seawater Sr isotope record for the upper Paleocene and lower Eocene. INTRODUCTION The Paleogene was a climatically dynamic period. Various climate proxies reveal a complex history of warming and cooling, characterized by periods of both gradual and rapid change (Miller et al., 1987; Miller and Katz, 1987; Stott et al., 1990; Zachos et al., 1994, 2001). Major events include a 1-m.y.-long global warming trend that began in the late Paleocene and climaxed in the early Eocene in a 1to 2-m.y.-long climatic optimum (early Eocene Climatic Optimum [EECO]) and a 12m.y.-long stepped cooling trend that began in the early middle Eocene and culminated in the earliest Oligocene with the appearance of continental-scale ice sheets (Hambrey et al., 1991; Zachos et al., 1992). One of the more prominent events is a transient but extreme greenhouse interval known as the Paleocene/Eocene Thermal Maximum (PETM) at ~55.0 Ma. Major changes in ocean chemistry, as inferred from carbon isotope anomalies, and changes in the distribution and preservation patterns of terrigenous and biogenic sediments on the seafloor (e.g., Bralower et al., 1995; Kennett and Stott, 1991; Robert and Kennett, 1997) characterize the PETM. In addition, distinct shifts in the distribution of key groups of fauna and flora occurred in the oceans and on land (e.g., Kelly et al., 1998; Koch et al., 1992, 1995; Thomas and Shackleton, 1996; Thomas, 1998; Wing, 1998). Another notable event is the earliest Oligocene Glacial Maximum (EOGM, or Oi-1), a brief but extreme glacial interval that occurred at ~33.4 Ma and marks the transition to permanent glacial conditions on Antarctica (e.g., Miller et al., 1987, 1991; Zachos et al., 1996; Coxall et al., 2005). This event, like the PETM, caused large-scale perturbations in ocean chemistry and paleoecology (Barrera and Huber, 1991, 1993; Salamy and Zachos, 1999; Thomas and Gooday, 1996; Thunell and Corliss, 1986). Multiple hypotheses exist to explain the large-scale, long-term changes in Paleogene climate, although none have yet gained universal acceptance. In general, among many factors, the role of ocean gateways (continental geography) and greenhouse gas levels are considered key variables. Theoretical models invoke either the absence of a circum-Antarctic current or higher greenhouse levels or some combination of both to account for the EECO (Barron, 1985; Bice et al., 2000; Sloan and Barron, 1992; Sloan and Rea, 1996; Sloan et al., 1992, 1995). Similarly, Oligocene glaciation has been attributed to both the initiation of the Antarctic Circumpolar Current (ACC) and a reduction in greenhouse gas levels (e.g., Kennett and Shackleton, 1976; Mikolajewicz et al., 1993; Oglesby, 1991; Raymo et al., 1990; Rind and Chandler, 1991; DeConto and Pollard, 2003). Some of the more abrupt transient excursions are more likely to have been forced by rapid changes in greenhouse gas levels because they occur over short timescales (e.g., 103–104 yr) and, most importantly, are accompanied by geochemical and isotopic anomalies suggestive of major perturbations in the carbon and sulfur cycles (Dickens et D. KROON ET AL. LEG 208 SYNTHESIS: CENOZOIC CLIMATE CYCLES AND EXCURSIONS 3 al., 1995, 1997; Paytan et al., 1998; Pearson and Palmer, 2000; Schmitz et al., 1997; Stott et al., 1990; Zachos et al., 1993). Further progress in characterizing Paleogene oceanography and climate history, particularly the transient events and rapid shifts, was slowed by the lack of high-quality, high-resolution, multicored sequences. Most sites cored prior to Leg 198 suffer from poor recovery and drilling disturbance, and few were multicored or drilled as part of depth transects. The few exceptions are sites recovered during recent Ocean Drilling Program (ODP) legs including Sites 865, 999, 1001, 1051, and Bass River core hole in New Jersey (Bralower et al., 1995, 1997; Miller et al., 1998; Norris and Röhl, 1999; Röhl et al., 2000, 2001, 2003). High-resolution records produced from these sites have yielded a wealth of exciting, important evidence of climate change to be more fully explored with additional data. The ODP extreme climate advisory panel (Program Planning Group [PPG]) recognized the dearth of highresolution records across climate transients, and the panel formulated new questions concerning extreme climates (Kroon et al., 2000) and potential drilling targets, among which was the Walvis Ridge area. Leg 208 was designed specifically to address this deficiency with a major goal of developing the high-fidelity records necessary to characterize shortterm events, including the changes in ocean chemistry and circulation and biota that theoretically should have accompanied these climatic extremes. Walvis Ridge, located in the eastern South Atlantic Ocean (Fig. F1), was one of the few known locations where previous drilling recovered the PETM and EOGM over a broad depth range. The ridge was the target of drilling by Deep Sea Drilling Project (DSDP) Leg 74, which occupied Sites 525–529 on the northern flank of the ridge at water depths between 2.5 and 4.2 km (Moore, Rabinowitz, et al., 1984). Paleogene pelagic sediments characterized by moderate sedimentation rates (~6–15 m/m.y.) and good magnetic stratigraphy were recovered at each site. However, because of poor recovery (~50%–75%) and coring disturbance, especially with the rotary core barrel in unlithified sediments, only short segments of the sequences were recovered fully intact and none of the sequences were double cored. Technical problems combined with the lack of high-resolution shipboard core logs limited highresolution cyclostratigraphic investigations to a few short segments of the Cretaceous/Paleogene (K/Pg) boundary interval (Herbert and D’Hondt, 1990). Nevertheless, subsequent shore-based studies of lowresolution samples collected from these cores were instrumental in adding to our understanding of long-term Maastrichtian and Paleogene paleoceanography of the South Atlantic Ocean (e.g., calcite compensation depth [CCD], carbon isotope stratigraphy, and deep-sea temperature/ice volume) (e.g., Moore, Rabinowitz, et al., 1984; Hsü, Labrecque, et al., 1984; Shackleton, 1987). Nearly complete PETM intervals were recovered at the shallowest and deepest Sites 525 and 527, respectively. Stable isotope analysis of foraminifers recovered from these sites helped constrain the magnitude of the deep Atlantic biogeochemical and environmental changes during this event (Thomas et al., 1999; Thomas and Shackleton, 1996). At the remaining sites, the PETM was not recovered because of core gaps. During the winter of 2000, a seismic survey of southeastern Walvis Ridge was carried out by the Meteor (Cruise M49/1; Speiss et al., 2003) (Fig. F2). The survey extended coverage of the Leg 74 sites to the north and northeast, where more continuous and slightly thicker sediment sequences were discovered. The higher-fidelity multichannel seismic (MCS) data gene

Emendation of the genus Streptochilus (Foraminifera) and new species from the lower Miocene of the Atlantic and Indian Oceans

Abstract: Three new species of Streptochilus , a biserial planktic foraminiferal genus, were recognized in the lower Miocene of the eastern Atlantic and western Indian Oceans. These species had been formerly thought to be benthic species of the genus Bolivina , but evidence on their apertural morphology and stable isotopic composition indicates that they lived as plankton and should be assigned to the genus Streptochilus . The observation that three morphological species occurred in different regions of the oceans during the same short period of time (18.9-17.2 Ma) suggests that these biserial planktic species may have evolved polyphyletically, either from biserial planktic or from benthic ancestors, possibly in response to the occurrence of relatively eutrophic environmental conditions caused by intermittent upwelling, leading to high algal growth rates but low transport efficiency of organic matter to the sea floor. The new species of Streptochilus are described, illustrated and named: S. rockallkiddensis sp. nov. (from the northeastern Atlantic), S. cetacensis sp. nov. (from the equatorial and southeastern Atlantic) and S. mascarenensis sp. nov. (from the western equatorial Indian Ocean) and the description of the genus is emended.