The Paleocene-Eocene Thermal Maximum (PETM)(55 Ma), was characterized by extreme global warming, ... more The Paleocene-Eocene Thermal Maximum (PETM)(55 Ma), was characterized by extreme global warming, a negative carbon isotope excursion, intensified carbonate dissolution, and a severe mass extinction of benthic foraminifera. The lack of continuous, undisturbed cores over a wide depth range has limited efforts to place constraints on key aspects of the PETM event, such as changes in ocean redox and carbon chemistry, and depth dependent patterns of the benthic extinction. The PE boundary was recovered in ...
The Paleocene-Eocene Thermal Maximum (PETM; aka LPTM) has been linked to a prominent perturbation... more The Paleocene-Eocene Thermal Maximum (PETM; aka LPTM) has been linked to a prominent perturbation in the global carbon cycle as evidenced by a~ 2.5 to 5.0% negative carbon isotope excursion (CIE) in marine and terrestrial carbon reservoirs, and a shoaling of the marine CCD. These changes have been attributed to rapid dissociation of a large mass of methane hydrate from the seafloor. Current efforts have focused on constraining both the amount and rate of methane carbon input, as well as the mechanism/trigger for release. ...
We report the results of new stable oxygen and carbon isotope analyses on carbonate fine fraction... more We report the results of new stable oxygen and carbon isotope analyses on carbonate fine fraction, whole rock, and benthic foraminifers, CaCO3 and coarse fraction percentage determinations, and trace element (Sr) analyses on carbonate constituents across the Cretaceous/Tertiary (K/T) boundary in Deep Sea Drilling Project (DSDP) sites 47.2, 356, 384, and 577 and compare them with published results from K/T boundary sections in other DSDP sites. We used the trace element data and scanning electron microscope examination to evaluate possible diagenetic alteration and relative preservation of the samples analyzed in this study. The ∂18O data when interpreted as isotopic paleotemperatures indicate relative stable surface water and deepwater temperatures in the late Maestrichtian followed by somewhat fluctuating temperatures in the early Paleocene. However, there is no indication of either a significant warming or cooling at or following the K/T boundary. Several sites do exhibit somewhat...
Fatty acids were measured in G. galathean um grown either phototrophically, or mixotrophically wi... more Fatty acids were measured in G. galathean um grown either phototrophically, or mixotrophically with Storeatula major (Cryptophyceae) as prey. G. galatheanum , like many photosynthetic dinoflagellates, contains high amounts of n-3 long-chain-polyunsaturated fatty acids (LC-PUFA) such as docosahexaenoic acid (DHA, 22:6n-3) and the hemolytic toxic fatty acid 18:5n-3. We hypothesize that a benefit of phagotrophy in G. galatheanum is the acquisition of precursor linolenic acid (18:3n-3) that fuels LC-PUFA synthesis. Phototrophs grew at 0.37 d Ϫ 1 , while mixotrophs grew at 0.40 d Ϫ 1 with a feeding rate of 0.62 d Ϫ 1. Photosynthesis was lower in mixotrophs (3.7 pg C cell Ϫ 1 h Ϫ 1) than phototrophs (4.9 pg C cell Ϫ 1 h Ϫ 1). DHA levels were higher in mixotrophs [3.7 (ϩ / Ϫ 0.11) pg cell Ϫ 1 ] than phototrophs [3.0 (ϩ / Ϫ 0.16) pg cell Ϫ 1 ] and prey [0.4 (ϩ / Ϫ 0.01) pg cell Ϫ 1 ]. 18:5n-3 levels [1.7 (ϩ / Ϫ 0.03) pg cell Ϫ 1 ] were similar in phototrophs and mixotrophs. An intermediate in n-3 LC-PUFA synthesis, 20:4n-3, accumulated in mixotrophs [0.6 (ϩ / Ϫ 0.27) pg cell Ϫ 1 ] relative to phototrophs (not detected) and prey [0.03 (ϩ / Ϫ 0.002) pg cell Ϫ 1 ]. Low ratios of linolenic acid to DHA in phototrophic G. galatheanum (0.14) relative to mixotrophic G. galatheanum (0.29) and prey (2.14) are consistent with substrate limitation of LC-PUFA synthesis in phototrophs. Accumulation of 20:4n-3 suggests incomplete conversion of linolenic acid to DHA, possibly due to conditions in batch culture. We conclude that precursors for n-3 LC-PUFA biosynthesis in G. galatheanum may be acquired through ingestion of S. major , and may partially control feeding/photosynthesis in mixotrophic populations.
We investigated rhodoliths (coralline red algae) from a subtropical locality in the Gulf of Calif... more We investigated rhodoliths (coralline red algae) from a subtropical locality in the Gulf of California (Lithotharnniurn crassiusculurn) and a subarctic locality in Newfoundland (Lithotharnniurn glaciale) for their potential as paleoenvironmental archives using microanalytical geochemical techniques to measure variations in ;5•80, Mg, and Ca. Rhodoliths are potentially well suited as recorders of shallow water paleoenvironmental signals because they (1) have worldwide distribution from the tropics to polar regions, (2) are long lived from decades to centuries, and (3) display well-developed growth bands. Our results indicate that rhodolith growth bands preserve ultrahigh-resolution records of paleoceanographicpaleoclimatic change and likely constitute an important new archive for reconstructing the paleoenvironmental history of littoral-neritic areas in which these algae are found. The ;5180 content of individually sampled rhodolith growth bands ranges from-2.4 to-4.6 %o in L. crassiusculurn and from-3.2 to-0.3 %o in L. glaciale. In both cases, the range of ;5•80 values suggests a slightly lower amplitude of variation in sea surface temperature than that actually measured in the ocean at the two study sites. Both L. crassiusculurn and L. glaciale show a negative offset from isotopic equilibrium. Electron microprobe analysis of magnesium and calcium in growth bands reveals cyclic variations with values ranging between 7.7-18.5 mol % MgCO3 in L. glaciale and 13.2-22.5 mol % MgCO3 in L. crassiusculurn. In addition, electron microprobe element maps highlight individual growth bands, provide a powerful approach to study rhodolith formation, and indicate that the specimens we analyzed have vertical growth rates of 250-450 •tm/yr. Reichart et al., 1998]. Studies of the oxygen isotopic composition of the aragonitic shell of A. islandica [Weidman et al., 1994] have confirmed the annual periodicity of its growth bands and the fact that it deposits its shell in isotopic equilibrium with the seawater. Although A. islandica can provide significant pa
The initiation of Antarctic glaciation in the early Oligocene (~34 Ma) is represented by a distin... more The initiation of Antarctic glaciation in the early Oligocene (~34 Ma) is represented by a distinct positive anomaly in the marine d 18 O record designated Oi-1 and accompanied by positive excursions in the mean d 13 C of oceanic dissolved inorganic carbon and biogenic sediment accumulation rates. Within 400 ky of the onset of Oi-1, the climate system settled into a more moderate but stable bglacialQ state. Here, through modeling, we investigate two of the principal biogeochemical processes involved in this response: silicate weathering and marine organic carbon cycling. We initiate the event with a rapid drawdown in atmospheric CO 2 resulting from increased weatherability of the continents associated with Himalayan orogeny. This perturbation triggers the overshoot and adjustment of the d 18 O record because of feedback among ice-sheet coverage, silicate weathering rates, and atmospheric CO 2. The system is a damped oscillator, the strength of which depends on the sensitivity of chemical weathering rates to climate change and climate to changes in atmospheric CO 2. Increased oceanic mixing associated with initial transition into a glacial world accelerates the rates of biological productivity and carbon burial, lowering atmospheric CO 2 and accelerating global cooling and ice-sheet growth, and generating a carbon isotope response that crudely approximates that observed. The Oi-1 overshoot appears to require a rapid (b1 million year) application of the forcing (e.g., tectonic drawdown of atmospheric CO 2). Although further investigation and more sophisticated models ultimately may show that other triggers and feedbacks prevailed during Oi-1, the modeling presented here demonstrates that simple feedbacks in the climate system can explain the overshoot and adjustment response to early Oligocene climate forcing.
T he PETM represents one of the largest and most abrupt greenhouse warming events in Earth's hist... more T he PETM represents one of the largest and most abrupt greenhouse warming events in Earth's history. Marine and terrestrial records document a global >2.5‰ negative carbon isotope excursion (CIE; refs 1-3) coincident with global mean surface ocean warming of >4 °C (ref. 4) and geochemical and sedimentological evidence for ocean acidification 5,6. Collectively, these lines of evidence suggest a rapid (10 3-10 4 years) and massive (~3,000-10,000 PgC) release of 13 C-depleted carbon into the ocean-atmosphere system 7-9. The PETM thus offers the opportunity to examine the response and recovery of the global carbon cycle and seawater carbonate chemistry to an ancient CO 2 release similar in magnitude to ongoing anthro pogenic fossil fuel combustion 10. Current understanding of long-term carbon cycle processes predicts that a rapid carbon injection should cause a short-term (0 to tens of thousands of years (kyr)) period of ocean acidification featuring reduced seawater carbonate saturation (Ω), followed by a longer-term (10-100 kyr) period of carbonate oversaturation due to elevated rates of terrestrial silicate weathering (see Box 1). This carbonate saturation overshoot manifests itself in several carbon cycle model simulations of the PETM 9,11 as an over-deepening of the CCD relative to its pre-event depth. Yet no records exist from abyssal sites below the pre-PETM CCD with which to detect possible CCD over-deepening. Sedimentary records from above the CCD (Southern Ocean Site 690
The boron to calcium ratio (B/Ca) in biogenic CaCO 3 is being increasingly utilized as a proxy fo... more The boron to calcium ratio (B/Ca) in biogenic CaCO 3 is being increasingly utilized as a proxy for past ocean carbonate chemistry. However, B/Ca of cultured and core-top foraminifers show dependence on multiple physicochemical seawater properties and only a few of those have been inorganically tested for their impacts. Accordingly, our understanding of the controls on foraminiferal B/Ca and thus how to interpret B/Ca in fossil shells is incomplete. To gain a clearer understanding of the B incorporation mechanism, we performed inorganic calcite precipitation experiments using a pH-stat system. As previously reported, we confirm that B/Ca in calcite increases with both fluid pH and total B concentration (denoted as [B T ], where [B T ] = [B(OH) 3 ] + [B(OH) 4 À ]). We provide the first evidence that B/Ca also increases with the concentration of total dissolved inorganic carbon (DIC) and calcium ion. With the exception of the [B T ] experiments, these chemical manipulations were accompanied by an increase in calcite saturation, and accordingly precipitation rate (denoted as R). But when pH and [Ca 2+ ] were jointly varied at a fixed saturation level to maintain relatively constant R at different pH and [Ca 2+ ] combinations, B/Ca was insensitive to both pH and [Ca 2+ ] changes. These experimental results unequivocally suggest kinetic effects related to R on B/Ca. Furthermore, with a suite of chemical manipulations we found that the B/Ca variability is explicable by just R and the [B T ]/[DIC] ratio in the parent fluids. This observation was particularly robust for relatively rapidly precipitated samples, whereas for relatively slowly precipitated samples, it was somewhat ambiguous whether the [B T ]/[DIC] or [B(OH) 4 À ]/[HCO 3 À ] ratio provides a better fit to the experimental data. Nonetheless, our experimental results can be considered as indirect evidence for incorporation of both B(OH) 4 À and B(OH) 3 into calcite. We propose a simple mathematical expression to describe the mode of B incorporation into synthetic calcite that depends only on the fluid [B T ]/[DIC] ratio and the precipitation rate R. This novel finding has important implications for future calibrations and applications of the B/Ca proxy as well as the d 11 B paleo-pH proxy.
Proceedings of the Ocean Drilling Program Scientific Results, 2006
The transition from the late Oligocene warm period into the early Miocene was marked by a series ... more The transition from the late Oligocene warm period into the early Miocene was marked by a series of rapid and brief episodes of cryospheric expansion and global cooling. We analyzed benthic foraminifers from nannofossil oozes recovered at Ocean Drilling Program Site 1218 to construct a stable isotope stratigraphy for the deep Pacific.
The meeting on Early Paleogene Warm Climates and Biosphere Dynamics was held June 9-13, 1999, in ... more The meeting on Early Paleogene Warm Climates and Biosphere Dynamics was held June 9-13, 1999, in G/3teborg, Sweden, and was organized and hosted by B. Schmitz. Schmitz was assisted by organizing committee members M.-P. Aubry and J. Zachos. This meeting was the fourth such gathering in recent years and was the largest and most successful meeting yet. There were 120 scientists in attendance from 21 countries. The largest group came from the United States; other wellrepresented countries were the United Kingdom, Russia, and Germany. The meeting, aimed at a multifaceted understanding of the early Paleogene, had a strong interdisciplinary character. Over 4 days, participants presented scientific research in oral and poster sessions. The meeting ended with a 1-day field trip to the early Paleogene deposits of northern Denmark. The early Paleogene may contain unique and critical records of greenhouse world conditions that could provide insights into other past and future greenhouse worlds. Results presented at this meeting reinforced this concept and showcased many exciting scientific developments. Great breadth and depth of information about conditions, theories, and predictions of Paleogene climatic and biotic systems were presented. New and important data were presented for the global oceans of the Paleogene as well as for continental sites located in Europe,
We have compiled the first stratigraphically continuous high-resolution benthic foraminiferal sta... more We have compiled the first stratigraphically continuous high-resolution benthic foraminiferal stable isotope record for the Paleocene from a single site utilizing cores recovered at Pacific ODP Site 1209. The long-term trend in the benthic isotope record suggests a close coupling of volcanic CO 2 input and deep-sea warming. Over the short-term the record is characterized by slow excursions with a pronounced periodic beat related to the short (100 kyr) and long (405 kyr) eccentricity cycle. The phase relationship between the benthic isotope record and eccentricity is similar to patterns documented for the Oligocene and Miocene confirming the role of orbital forcing as the pace maker for paleoclimatic variability on Milankovitch time scales. In addition, the record documents an unusual transient warming of 2°C coeval with a 0.6‰ carbon isotope excursion and a decrease in carbonate content at 61.75 Ma. This event, which bears some resemblance to Eocene hyperthermals, marks the onset of a long-term decline in d 13 C. The timing indicates it might be related to the initiation of volcanism along Greenland margin.
Characterizing polar climates during past warm intervals is important for understanding`greenhous... more Characterizing polar climates during past warm intervals is important for understanding`greenhouse' climate dynamics because high-latitude surface temperatures and precipitation patterns are extremely sensitive to global climatic conditions. Model-data comparisons of high-latitude climates during past warm intervals (Cretaceous±Eocene, Pliocene) are currently at odds. Speci®cally, simulations of past warm climates produce polar regions characterized by sub-freezing temperatures and signi®cant seasonality, whereas limited fossil proxy data indicate higher mean annual temperatures and low seasonality (i.e. an equable climate). We have constructed a data set to infer northern hemisphere polar marine temperatures during the late Paleocene. Seasonal and mean annual temperature and the oxygen isotopic composition of precipitation are reconstructed for an Arctic coastal setting during the Thanetian (57±58 Ma) using fossil shell stable isotope pro®les. We estimate that coastal water temperatures varied between 10 and 158C during the seasons of growth, presumably spring, summer and fall. These ®ndings support paleontological evidence, implying Northern Hemisphere polar climates were seasonally warm during the late Paleocene. In addition, estuarine fossil oxygen isotope pro®les show periodic excursions to low values (as low as 219½ VPDB), which indicate seasonal pulses of isotopically-depleted freshwater.
A perturbation of the carbon cycle and biosphere, linked to globally increased temperatures about... more A perturbation of the carbon cycle and biosphere, linked to globally increased temperatures about 55.9 million years ago, characterized the Palaeocene-Eocene Thermal Maximum. Its e ect on global oceanic productivity is controversial. Here we present records of marine barite accumulation rates that show distinct peaks during this time interval, suggesting a general increase in export productivity. We propose that changes in marine ecosystems, resulting from high atmospheric partial pressure of CO 2 and ocean acidification, led to enhanced carbon export from the photic zone to depth, thereby increasing the e ciency of the biological pump. Higher seawater temperatures at that time increased bacterial activity and organic matter regeneration. Through this process much of the sinking particulate organic matter was probably converted to dissolved inorganic and organic carbon. We estimate that an annual carbon export flux out of the euphotic zone and into the deep ocean waters could have amounted to about 15 Gt during the Palaeocene-Eocene Thermal Maximum. About 0.4% of this carbon is expected to have entered the refractory dissolved organic pool, where it could be sequestered from the atmosphere for tens of thousands of years. Our estimates are consistent with the amount of carbon redistribution expected for the recovery from the Palaeocene-Eocene Thermal Maximum.
A series of transient global warming events 1,2 occurred during the late Palaeocene and early Eoc... more A series of transient global warming events 1,2 occurred during the late Palaeocene and early Eocene, about 59 to 50 million years ago. The events, although variable in magnitude, were apparently paced by orbital cycles 2-4 and linked to massive perturbations of the global carbon cycle 5,6. However, a causal link between orbital changes in insolation and the carbon cycle has yet to be established for this time period. Here we present a series of coupled climate model simulations that demonstrate that orbitally induced changes in ocean circulation and intermediate water temperature can trigger the destabilization of methane hydrates. We then use a simple threshold model to show that progressive global warming over millions of years, in combination with the increasing tendency of the ocean to remain in a more stagnant state, can explain the decreasing magnitude and increasing frequency of hyperthermal events throughout the early Eocene. Our work shows that nonlinear interactions between climate and the carbon cycle can modulate the effect of orbital variations, in this case producing transient global warming events with varying timing and magnitude. From the late Palaeocene to the early Eocene (∼59-50 Myr), Earth's surface and oceans went through an interval of progressive warming, culminating in the early Eocene climatic optimum (EECO, ∼51 Myr; ref. 6; Fig. 1). Superimposed on this gradual warming trend are a series of 'hyperthermal' events-geologically abrupt (<10 kyr) warmings of Earth's surface and deep ocean, the most prominent of which being the Palaeocene-Eocene Thermal Maximum (PETM, ∼56 Myr; ref. 6). Two subsequent smaller events, ETM2 (∼54 Myr; ref. 2), and ETM3 (∼53 Myr; ref. 7) seem to share similar characteristics. Associated with the hyperthermals are large negative carbon-isotope excursions of surficial carbon reservoirs and dissolution of deep-sea carbonate (Fig. 1; refs. 8), consistent with massive injections of 13 Cdepleted carbon into the ocean-atmosphere system. Proposed sources for this carbon include methane hydrates 5 , terrestrial peat deposits 9 , and thermogenic methane 10. Understanding the causes of hyperthermals is important, not least because the amount of the carbon release and magnitude of global warming can be used to estimate climate sensitivity. The hyperthermals also reflect possible threshold events ('tipping points'), with slow changes in boundary conditions giving rise to rapid positive feedback and state transitions in the carbon-climate system 11-13. Cyclostratigraphic evidence from complete marine sections suggest that the PETM, ETM2, and ETM3 events all initiated on maxima in the 100 kyr eccentricity cycle 2-4. This has led to the suggestion that orbital pacing controlled the timing of carbon injection 2,4 and hence the occurrence of the hyperthermal events.
The Paleocene-Eocene Thermal Maximum (PETM)(55 Ma), was characterized by extreme global warming, ... more The Paleocene-Eocene Thermal Maximum (PETM)(55 Ma), was characterized by extreme global warming, a negative carbon isotope excursion, intensified carbonate dissolution, and a severe mass extinction of benthic foraminifera. The lack of continuous, undisturbed cores over a wide depth range has limited efforts to place constraints on key aspects of the PETM event, such as changes in ocean redox and carbon chemistry, and depth dependent patterns of the benthic extinction. The PE boundary was recovered in ...
The Paleocene-Eocene Thermal Maximum (PETM; aka LPTM) has been linked to a prominent perturbation... more The Paleocene-Eocene Thermal Maximum (PETM; aka LPTM) has been linked to a prominent perturbation in the global carbon cycle as evidenced by a~ 2.5 to 5.0% negative carbon isotope excursion (CIE) in marine and terrestrial carbon reservoirs, and a shoaling of the marine CCD. These changes have been attributed to rapid dissociation of a large mass of methane hydrate from the seafloor. Current efforts have focused on constraining both the amount and rate of methane carbon input, as well as the mechanism/trigger for release. ...
We report the results of new stable oxygen and carbon isotope analyses on carbonate fine fraction... more We report the results of new stable oxygen and carbon isotope analyses on carbonate fine fraction, whole rock, and benthic foraminifers, CaCO3 and coarse fraction percentage determinations, and trace element (Sr) analyses on carbonate constituents across the Cretaceous/Tertiary (K/T) boundary in Deep Sea Drilling Project (DSDP) sites 47.2, 356, 384, and 577 and compare them with published results from K/T boundary sections in other DSDP sites. We used the trace element data and scanning electron microscope examination to evaluate possible diagenetic alteration and relative preservation of the samples analyzed in this study. The ∂18O data when interpreted as isotopic paleotemperatures indicate relative stable surface water and deepwater temperatures in the late Maestrichtian followed by somewhat fluctuating temperatures in the early Paleocene. However, there is no indication of either a significant warming or cooling at or following the K/T boundary. Several sites do exhibit somewhat...
Fatty acids were measured in G. galathean um grown either phototrophically, or mixotrophically wi... more Fatty acids were measured in G. galathean um grown either phototrophically, or mixotrophically with Storeatula major (Cryptophyceae) as prey. G. galatheanum , like many photosynthetic dinoflagellates, contains high amounts of n-3 long-chain-polyunsaturated fatty acids (LC-PUFA) such as docosahexaenoic acid (DHA, 22:6n-3) and the hemolytic toxic fatty acid 18:5n-3. We hypothesize that a benefit of phagotrophy in G. galatheanum is the acquisition of precursor linolenic acid (18:3n-3) that fuels LC-PUFA synthesis. Phototrophs grew at 0.37 d Ϫ 1 , while mixotrophs grew at 0.40 d Ϫ 1 with a feeding rate of 0.62 d Ϫ 1. Photosynthesis was lower in mixotrophs (3.7 pg C cell Ϫ 1 h Ϫ 1) than phototrophs (4.9 pg C cell Ϫ 1 h Ϫ 1). DHA levels were higher in mixotrophs [3.7 (ϩ / Ϫ 0.11) pg cell Ϫ 1 ] than phototrophs [3.0 (ϩ / Ϫ 0.16) pg cell Ϫ 1 ] and prey [0.4 (ϩ / Ϫ 0.01) pg cell Ϫ 1 ]. 18:5n-3 levels [1.7 (ϩ / Ϫ 0.03) pg cell Ϫ 1 ] were similar in phototrophs and mixotrophs. An intermediate in n-3 LC-PUFA synthesis, 20:4n-3, accumulated in mixotrophs [0.6 (ϩ / Ϫ 0.27) pg cell Ϫ 1 ] relative to phototrophs (not detected) and prey [0.03 (ϩ / Ϫ 0.002) pg cell Ϫ 1 ]. Low ratios of linolenic acid to DHA in phototrophic G. galatheanum (0.14) relative to mixotrophic G. galatheanum (0.29) and prey (2.14) are consistent with substrate limitation of LC-PUFA synthesis in phototrophs. Accumulation of 20:4n-3 suggests incomplete conversion of linolenic acid to DHA, possibly due to conditions in batch culture. We conclude that precursors for n-3 LC-PUFA biosynthesis in G. galatheanum may be acquired through ingestion of S. major , and may partially control feeding/photosynthesis in mixotrophic populations.
We investigated rhodoliths (coralline red algae) from a subtropical locality in the Gulf of Calif... more We investigated rhodoliths (coralline red algae) from a subtropical locality in the Gulf of California (Lithotharnniurn crassiusculurn) and a subarctic locality in Newfoundland (Lithotharnniurn glaciale) for their potential as paleoenvironmental archives using microanalytical geochemical techniques to measure variations in ;5•80, Mg, and Ca. Rhodoliths are potentially well suited as recorders of shallow water paleoenvironmental signals because they (1) have worldwide distribution from the tropics to polar regions, (2) are long lived from decades to centuries, and (3) display well-developed growth bands. Our results indicate that rhodolith growth bands preserve ultrahigh-resolution records of paleoceanographicpaleoclimatic change and likely constitute an important new archive for reconstructing the paleoenvironmental history of littoral-neritic areas in which these algae are found. The ;5180 content of individually sampled rhodolith growth bands ranges from-2.4 to-4.6 %o in L. crassiusculurn and from-3.2 to-0.3 %o in L. glaciale. In both cases, the range of ;5•80 values suggests a slightly lower amplitude of variation in sea surface temperature than that actually measured in the ocean at the two study sites. Both L. crassiusculurn and L. glaciale show a negative offset from isotopic equilibrium. Electron microprobe analysis of magnesium and calcium in growth bands reveals cyclic variations with values ranging between 7.7-18.5 mol % MgCO3 in L. glaciale and 13.2-22.5 mol % MgCO3 in L. crassiusculurn. In addition, electron microprobe element maps highlight individual growth bands, provide a powerful approach to study rhodolith formation, and indicate that the specimens we analyzed have vertical growth rates of 250-450 •tm/yr. Reichart et al., 1998]. Studies of the oxygen isotopic composition of the aragonitic shell of A. islandica [Weidman et al., 1994] have confirmed the annual periodicity of its growth bands and the fact that it deposits its shell in isotopic equilibrium with the seawater. Although A. islandica can provide significant pa
The initiation of Antarctic glaciation in the early Oligocene (~34 Ma) is represented by a distin... more The initiation of Antarctic glaciation in the early Oligocene (~34 Ma) is represented by a distinct positive anomaly in the marine d 18 O record designated Oi-1 and accompanied by positive excursions in the mean d 13 C of oceanic dissolved inorganic carbon and biogenic sediment accumulation rates. Within 400 ky of the onset of Oi-1, the climate system settled into a more moderate but stable bglacialQ state. Here, through modeling, we investigate two of the principal biogeochemical processes involved in this response: silicate weathering and marine organic carbon cycling. We initiate the event with a rapid drawdown in atmospheric CO 2 resulting from increased weatherability of the continents associated with Himalayan orogeny. This perturbation triggers the overshoot and adjustment of the d 18 O record because of feedback among ice-sheet coverage, silicate weathering rates, and atmospheric CO 2. The system is a damped oscillator, the strength of which depends on the sensitivity of chemical weathering rates to climate change and climate to changes in atmospheric CO 2. Increased oceanic mixing associated with initial transition into a glacial world accelerates the rates of biological productivity and carbon burial, lowering atmospheric CO 2 and accelerating global cooling and ice-sheet growth, and generating a carbon isotope response that crudely approximates that observed. The Oi-1 overshoot appears to require a rapid (b1 million year) application of the forcing (e.g., tectonic drawdown of atmospheric CO 2). Although further investigation and more sophisticated models ultimately may show that other triggers and feedbacks prevailed during Oi-1, the modeling presented here demonstrates that simple feedbacks in the climate system can explain the overshoot and adjustment response to early Oligocene climate forcing.
T he PETM represents one of the largest and most abrupt greenhouse warming events in Earth's hist... more T he PETM represents one of the largest and most abrupt greenhouse warming events in Earth's history. Marine and terrestrial records document a global >2.5‰ negative carbon isotope excursion (CIE; refs 1-3) coincident with global mean surface ocean warming of >4 °C (ref. 4) and geochemical and sedimentological evidence for ocean acidification 5,6. Collectively, these lines of evidence suggest a rapid (10 3-10 4 years) and massive (~3,000-10,000 PgC) release of 13 C-depleted carbon into the ocean-atmosphere system 7-9. The PETM thus offers the opportunity to examine the response and recovery of the global carbon cycle and seawater carbonate chemistry to an ancient CO 2 release similar in magnitude to ongoing anthro pogenic fossil fuel combustion 10. Current understanding of long-term carbon cycle processes predicts that a rapid carbon injection should cause a short-term (0 to tens of thousands of years (kyr)) period of ocean acidification featuring reduced seawater carbonate saturation (Ω), followed by a longer-term (10-100 kyr) period of carbonate oversaturation due to elevated rates of terrestrial silicate weathering (see Box 1). This carbonate saturation overshoot manifests itself in several carbon cycle model simulations of the PETM 9,11 as an over-deepening of the CCD relative to its pre-event depth. Yet no records exist from abyssal sites below the pre-PETM CCD with which to detect possible CCD over-deepening. Sedimentary records from above the CCD (Southern Ocean Site 690
The boron to calcium ratio (B/Ca) in biogenic CaCO 3 is being increasingly utilized as a proxy fo... more The boron to calcium ratio (B/Ca) in biogenic CaCO 3 is being increasingly utilized as a proxy for past ocean carbonate chemistry. However, B/Ca of cultured and core-top foraminifers show dependence on multiple physicochemical seawater properties and only a few of those have been inorganically tested for their impacts. Accordingly, our understanding of the controls on foraminiferal B/Ca and thus how to interpret B/Ca in fossil shells is incomplete. To gain a clearer understanding of the B incorporation mechanism, we performed inorganic calcite precipitation experiments using a pH-stat system. As previously reported, we confirm that B/Ca in calcite increases with both fluid pH and total B concentration (denoted as [B T ], where [B T ] = [B(OH) 3 ] + [B(OH) 4 À ]). We provide the first evidence that B/Ca also increases with the concentration of total dissolved inorganic carbon (DIC) and calcium ion. With the exception of the [B T ] experiments, these chemical manipulations were accompanied by an increase in calcite saturation, and accordingly precipitation rate (denoted as R). But when pH and [Ca 2+ ] were jointly varied at a fixed saturation level to maintain relatively constant R at different pH and [Ca 2+ ] combinations, B/Ca was insensitive to both pH and [Ca 2+ ] changes. These experimental results unequivocally suggest kinetic effects related to R on B/Ca. Furthermore, with a suite of chemical manipulations we found that the B/Ca variability is explicable by just R and the [B T ]/[DIC] ratio in the parent fluids. This observation was particularly robust for relatively rapidly precipitated samples, whereas for relatively slowly precipitated samples, it was somewhat ambiguous whether the [B T ]/[DIC] or [B(OH) 4 À ]/[HCO 3 À ] ratio provides a better fit to the experimental data. Nonetheless, our experimental results can be considered as indirect evidence for incorporation of both B(OH) 4 À and B(OH) 3 into calcite. We propose a simple mathematical expression to describe the mode of B incorporation into synthetic calcite that depends only on the fluid [B T ]/[DIC] ratio and the precipitation rate R. This novel finding has important implications for future calibrations and applications of the B/Ca proxy as well as the d 11 B paleo-pH proxy.
Proceedings of the Ocean Drilling Program Scientific Results, 2006
The transition from the late Oligocene warm period into the early Miocene was marked by a series ... more The transition from the late Oligocene warm period into the early Miocene was marked by a series of rapid and brief episodes of cryospheric expansion and global cooling. We analyzed benthic foraminifers from nannofossil oozes recovered at Ocean Drilling Program Site 1218 to construct a stable isotope stratigraphy for the deep Pacific.
The meeting on Early Paleogene Warm Climates and Biosphere Dynamics was held June 9-13, 1999, in ... more The meeting on Early Paleogene Warm Climates and Biosphere Dynamics was held June 9-13, 1999, in G/3teborg, Sweden, and was organized and hosted by B. Schmitz. Schmitz was assisted by organizing committee members M.-P. Aubry and J. Zachos. This meeting was the fourth such gathering in recent years and was the largest and most successful meeting yet. There were 120 scientists in attendance from 21 countries. The largest group came from the United States; other wellrepresented countries were the United Kingdom, Russia, and Germany. The meeting, aimed at a multifaceted understanding of the early Paleogene, had a strong interdisciplinary character. Over 4 days, participants presented scientific research in oral and poster sessions. The meeting ended with a 1-day field trip to the early Paleogene deposits of northern Denmark. The early Paleogene may contain unique and critical records of greenhouse world conditions that could provide insights into other past and future greenhouse worlds. Results presented at this meeting reinforced this concept and showcased many exciting scientific developments. Great breadth and depth of information about conditions, theories, and predictions of Paleogene climatic and biotic systems were presented. New and important data were presented for the global oceans of the Paleogene as well as for continental sites located in Europe,
We have compiled the first stratigraphically continuous high-resolution benthic foraminiferal sta... more We have compiled the first stratigraphically continuous high-resolution benthic foraminiferal stable isotope record for the Paleocene from a single site utilizing cores recovered at Pacific ODP Site 1209. The long-term trend in the benthic isotope record suggests a close coupling of volcanic CO 2 input and deep-sea warming. Over the short-term the record is characterized by slow excursions with a pronounced periodic beat related to the short (100 kyr) and long (405 kyr) eccentricity cycle. The phase relationship between the benthic isotope record and eccentricity is similar to patterns documented for the Oligocene and Miocene confirming the role of orbital forcing as the pace maker for paleoclimatic variability on Milankovitch time scales. In addition, the record documents an unusual transient warming of 2°C coeval with a 0.6‰ carbon isotope excursion and a decrease in carbonate content at 61.75 Ma. This event, which bears some resemblance to Eocene hyperthermals, marks the onset of a long-term decline in d 13 C. The timing indicates it might be related to the initiation of volcanism along Greenland margin.
Characterizing polar climates during past warm intervals is important for understanding`greenhous... more Characterizing polar climates during past warm intervals is important for understanding`greenhouse' climate dynamics because high-latitude surface temperatures and precipitation patterns are extremely sensitive to global climatic conditions. Model-data comparisons of high-latitude climates during past warm intervals (Cretaceous±Eocene, Pliocene) are currently at odds. Speci®cally, simulations of past warm climates produce polar regions characterized by sub-freezing temperatures and signi®cant seasonality, whereas limited fossil proxy data indicate higher mean annual temperatures and low seasonality (i.e. an equable climate). We have constructed a data set to infer northern hemisphere polar marine temperatures during the late Paleocene. Seasonal and mean annual temperature and the oxygen isotopic composition of precipitation are reconstructed for an Arctic coastal setting during the Thanetian (57±58 Ma) using fossil shell stable isotope pro®les. We estimate that coastal water temperatures varied between 10 and 158C during the seasons of growth, presumably spring, summer and fall. These ®ndings support paleontological evidence, implying Northern Hemisphere polar climates were seasonally warm during the late Paleocene. In addition, estuarine fossil oxygen isotope pro®les show periodic excursions to low values (as low as 219½ VPDB), which indicate seasonal pulses of isotopically-depleted freshwater.
A perturbation of the carbon cycle and biosphere, linked to globally increased temperatures about... more A perturbation of the carbon cycle and biosphere, linked to globally increased temperatures about 55.9 million years ago, characterized the Palaeocene-Eocene Thermal Maximum. Its e ect on global oceanic productivity is controversial. Here we present records of marine barite accumulation rates that show distinct peaks during this time interval, suggesting a general increase in export productivity. We propose that changes in marine ecosystems, resulting from high atmospheric partial pressure of CO 2 and ocean acidification, led to enhanced carbon export from the photic zone to depth, thereby increasing the e ciency of the biological pump. Higher seawater temperatures at that time increased bacterial activity and organic matter regeneration. Through this process much of the sinking particulate organic matter was probably converted to dissolved inorganic and organic carbon. We estimate that an annual carbon export flux out of the euphotic zone and into the deep ocean waters could have amounted to about 15 Gt during the Palaeocene-Eocene Thermal Maximum. About 0.4% of this carbon is expected to have entered the refractory dissolved organic pool, where it could be sequestered from the atmosphere for tens of thousands of years. Our estimates are consistent with the amount of carbon redistribution expected for the recovery from the Palaeocene-Eocene Thermal Maximum.
A series of transient global warming events 1,2 occurred during the late Palaeocene and early Eoc... more A series of transient global warming events 1,2 occurred during the late Palaeocene and early Eocene, about 59 to 50 million years ago. The events, although variable in magnitude, were apparently paced by orbital cycles 2-4 and linked to massive perturbations of the global carbon cycle 5,6. However, a causal link between orbital changes in insolation and the carbon cycle has yet to be established for this time period. Here we present a series of coupled climate model simulations that demonstrate that orbitally induced changes in ocean circulation and intermediate water temperature can trigger the destabilization of methane hydrates. We then use a simple threshold model to show that progressive global warming over millions of years, in combination with the increasing tendency of the ocean to remain in a more stagnant state, can explain the decreasing magnitude and increasing frequency of hyperthermal events throughout the early Eocene. Our work shows that nonlinear interactions between climate and the carbon cycle can modulate the effect of orbital variations, in this case producing transient global warming events with varying timing and magnitude. From the late Palaeocene to the early Eocene (∼59-50 Myr), Earth's surface and oceans went through an interval of progressive warming, culminating in the early Eocene climatic optimum (EECO, ∼51 Myr; ref. 6; Fig. 1). Superimposed on this gradual warming trend are a series of 'hyperthermal' events-geologically abrupt (<10 kyr) warmings of Earth's surface and deep ocean, the most prominent of which being the Palaeocene-Eocene Thermal Maximum (PETM, ∼56 Myr; ref. 6). Two subsequent smaller events, ETM2 (∼54 Myr; ref. 2), and ETM3 (∼53 Myr; ref. 7) seem to share similar characteristics. Associated with the hyperthermals are large negative carbon-isotope excursions of surficial carbon reservoirs and dissolution of deep-sea carbonate (Fig. 1; refs. 8), consistent with massive injections of 13 Cdepleted carbon into the ocean-atmosphere system. Proposed sources for this carbon include methane hydrates 5 , terrestrial peat deposits 9 , and thermogenic methane 10. Understanding the causes of hyperthermals is important, not least because the amount of the carbon release and magnitude of global warming can be used to estimate climate sensitivity. The hyperthermals also reflect possible threshold events ('tipping points'), with slow changes in boundary conditions giving rise to rapid positive feedback and state transitions in the carbon-climate system 11-13. Cyclostratigraphic evidence from complete marine sections suggest that the PETM, ETM2, and ETM3 events all initiated on maxima in the 100 kyr eccentricity cycle 2-4. This has led to the suggestion that orbital pacing controlled the timing of carbon injection 2,4 and hence the occurrence of the hyperthermal events.
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Papers by J Zachos