scispace - formally typeset
Search or ask a question

Showing papers in "Paleoceanography in 2005"


Journal ArticleDOI
TL;DR: In this paper, a 53-Myr stack (LR04) of benthic δ18O records from 57 globally distributed sites aligned by an automated graphic correlation algorithm is presented.
Abstract: [1] We present a 53-Myr stack (the “LR04” stack) of benthic δ18O records from 57 globally distributed sites aligned by an automated graphic correlation algorithm This is the first benthic δ18O stack composed of more than three records to extend beyond 850 ka, and we use its improved signal quality to identify 24 new marine isotope stages in the early Pliocene We also present a new LR04 age model for the Pliocene-Pleistocene derived from tuning the δ18O stack to a simple ice model based on 21 June insolation at 65°N Stacked sedimentation rates provide additional age model constraints to prevent overtuning Despite a conservative tuning strategy, the LR04 benthic stack exhibits significant coherency with insolation in the obliquity band throughout the entire 53 Myr and in the precession band for more than half of the record The LR04 stack contains significantly more variance in benthic δ18O than previously published stacks of the late Pleistocene as the result of higher-resolution records, a better alignment technique, and a greater percentage of records from the Atlantic Finally, the relative phases of the stack's 41- and 23-kyr components suggest that the precession component of δ18O from 27–16 Ma is primarily a deep-water temperature signal and that the phase of δ18O precession response changed suddenly at 16 Ma

6,186 citations


Journal ArticleDOI
TL;DR: In this article, oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa Cibicidoides and Planulina from 25 new piston cores, gravity cores, and multicores from the Brazil margin.
Abstract: [1] Oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa Cibicidoides and Planulina from 25 new piston cores, gravity cores, and multicores from the Brazil margin. The cores span water depths from about 400 to 3000 m and intersect the major water masses in this region. These new data fill a critical gap in the South Atlantic Ocean and provide the motivation for updating the classic glacial western Atlantic δ13C transect of Duplessy et al. (1988). The distribution of δ13C of ΣCO2 requires the presence of three distinct water masses in the glacial Atlantic Ocean: a shallow (∼1000 m), southern source water mass with an end-member δ13C value of about 0.3–0.5‰ VPDB, a middepth (∼1500 m), northern source water mass with an end-member value of about 1.5‰, and a deep (>2000 m), southern source water with an end-member value of less than −0.2‰, and perhaps as low as the −0.9‰ values observed in the South Atlantic sector of the Southern Ocean (Ninnemann and Charles, 2002). The origins of the water masses are supported by the meridional gradients in benthic foraminiferal δ18O. A revised glacial section of deep water δ13C documents the positions and gradients among these end-member intermediate and deep water masses. The large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation.

523 citations


Journal ArticleDOI
TL;DR: In this paper, a high-resolution marine sediment record from the El Nino region off the coast of Peru spanning the last 20,000 years is presented, where sea surface temperature, photosynthetic pigments, and a lithic proxy for El-Nino flood events on the continent are used as paleo-El Nino-Southern Oscillation proxy data.
Abstract: Here we present a high-resolution marine sediment record from the El Nino region off the coast of Peru spanning the last 20,000 years. Sea surface temperature, photosynthetic pigments, and a lithic proxy for El Nino flood events on the continent are used as paleo–El Nino–Southern Oscillation proxy data. The onset of stronger El Nino activity in Peru started around 17,000 calibrated years before the present, which is later than modeling experiments show but contemporaneous with the Heinrich event 1. Maximum El Nino activity occurred during the early and late Holocene, especially during the second and third millennium B.P. The recurrence period of very strong El Nino events is 60–80 years. El Nino events were weak before and during the beginning of the Younger Dryas, during the middle of the Holocene, and during medieval times. The strength of El Nino flood events during the last millennium has positive and negative relationships to global and Northern Hemisphere temperature reconstructions.

378 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used 45 δ18O records from benthic and planktonic foraminifera and globally distributed sites to reconstruct the dynamics of the Northern Hemisphere glaciation (NHG) initiation.
Abstract: [1] Unraveling the dynamics of the Northern Hemisphere glaciation (NHG) in the Pliocene is a key step toward a quantitative theory of the climate transition from a greenhouse to an icehouse world. Extracting the ice volume signal from marine oxygen isotope (δ18O) records corrupted with “temperature noise” can be accomplished using statistical time series analysis. We use 45 δ18O records from benthic and planktonic foraminifera and globally distributed sites to reconstruct the dynamics of NHG initiation. We compare δ18O amplitudes with those of temperature proxy records and estimate a global ice volume–related increase of 0.4‰, equivalent to an overall sea level lowering of 43 m. We find the NHG started significantly earlier than previously assumed, as early as 3.6 Ma, and ended at 2.4 Ma. This long-term increase points to slow, tectonic forcing such as closing of ocean gateways or mountain building as the root cause of the NHG.

320 citations


Journal ArticleDOI
TL;DR: In this article, the authors determined the distribution of major, minor, and trace element abundances in the Rock Canyon marine sedimentary section (Pueblo, Colorado) and detected an interval of concentrated metal abundance anomalies that coincides with the abrupt beginning of the positive δ13C isotope excursion.
Abstract: [1] Rapid eruption of submarine lava flows during formation of the Caribbean plateau correlates closely with ocean anoxic event 2 (OAE2), which bracketed the Cenomanian/Turonian boundary (∼935 Ma) These events also correspond with a positive excursion in carbon isotopic composition of seawater Hydrothermal activity associated with large-scale submarine volcanism may have been responsible for this abrupt change in ocean chemistry We determined the distribution of major, minor, and trace element abundances in the Rock Canyon marine sedimentary section (Pueblo, Colorado) After normalizing element concentrations to Zr to remove the variable contribution of terrigenous material to these sediments, we detected an interval of concentrated metal abundance anomalies that coincides with the abrupt beginning of the positive δ13C isotope excursion The metal abundance anomalies indicate that intermittent hydrothermal activity, in the form of both water/rock exchange and magmatic degassing, introduced large concentrations of trace metals into the Cretaceous ocean at the same time that extinctions of benthic species, turnover in plankton communities, and increases in isotopically light organic carbon burial occurred The stratigraphic position of this interval of trace metal anomalies matches events associated with OAE2 and indicates that intermittent hydrothermal activity on a massive scale triggered abrupt changes in carbon burial and deep ocean oxygen contents

206 citations


Journal ArticleDOI
TL;DR: In this article, high-resolution geochemical records from a depth transect through the Cenomanian/Turonian (C/T) Tarfaya Basin (northwest African Shelf) reveal high-amplitude fluctuations in accumulation rates of organic carbon (OC), redox-sensitive and sulphide-forming trace metals, and biomarkers indicative of photic zone euxinia.
Abstract: High-resolution geochemical records from a depth transect through the Cenomanian/Turonian (C/T) Tarfaya Basin (northwest African Shelf) reveal high-amplitude fluctuations in accumulation rates of organic carbon (OC), redox-sensitive and sulphide-forming trace metals, and biomarkers indicative of photic zone euxinia. These fluctuations are in general coeval and thus imply a strong relationship of OC burial and water column redox conditions. The pacing and regularity of the records and the absence of a prominent continental signature suggest a dynamic depositional setting linked to orbital and higher-frequency forcing. Determining the dominant frequency depends on the definition of the most pronounced oceanic anoxic event (OAE2) and its duration. We propose that eccentricity is the main forcing factor at Tarfaya and controlled fluctuations in wind-driven upwelling of nutrient-rich, oxygen-depleted intermediate waters from the adjacent Atlantic and the periodic development of photic zone and bottom water euxinia on the mid-Cretaceous northwest African shelf. Accumulation records clearly identify the basin center as the primary site of sediment deposition with highest temporal variability and an up to six-fold increase in OC burial from similar to2 g/m(2) . yr prior to the OAE2 to similar to12 g/m(2) . yr during the OAE2. Photic zone and bottom water euxinia alternated with periods of greater oxygenation of the water column in response to climate forcing. Mass balance calculations imply that similar to2% of the overall global excess OC burial associated with the OAE2 was deposited in the Tarfaya Basin, an area that represented only similar to0.05% of the total global C/T ocean floor. In fact, the lateral extent of similar black shales along the African continental margin indicates that this part of the ocean contributed significantly to the global increase in organic carbon burial during the OAE2.

194 citations


Journal ArticleDOI
TL;DR: The TEX86 is a temperature proxy which is based on the number of cyclopentane moieties in the Glycerol dialkyl glycerol tetraether (GDGT) lipids of Crenarchaeota that occur ubiquitously in oceans and shelf seas as mentioned in this paper.
Abstract: The TEX86 is a new temperature proxy which is based on the number of cyclopentane moieties in the glycerol dialkyl glycerol tetraether (GDGT) lipids of the membranes of Crenarchaeota that occur ubiquitously in oceans and shelf seas. This proxy was calibrated by core top sediments, but it is as yet not clear during which season and at which depth in the water column the GDGT signal used for TEX86 paleothermometry is biosynthesized. Here we analyzed >200 particulate organic matter (POM) samples from 11 different marine settings for TEX86. This revealed that the GDGTs occur seasonally in surface waters and occur in higher abundances during the winter and spring months. The depth distribution showed that GDGTs generally appeared in higher amounts below 100 m depth in the water column. However, the TEX86 values for waters below the photic zone (150–1500 m) did not correlate with the in situ temperature but rather correlated linearly with surface temperature. The TEX86 for POM from the upper 100 m showed a linear correlation with in situ temperature, which was nearly identical to the previously reported core top equation. The correlation of all POM samples with surface temperature was also strikingly similar to the core top correlation. These findings demonstrate that the GDGT signal which reaches the sediment is mainly derived from the upper 100 m of the water column. This may be caused by the fact that GDGTs from the photic zone are much more effectively transported to the sediment by grazing and repackaging in large particles than GDGTs from deeper waters.

191 citations


Journal ArticleDOI
TL;DR: In this article, the authors present C isotope records from belemnites (delta(13)C(bel)) sampled from two localities, calibrated with high-resolution ammonite biostratigraphy and Sr isotope stratigraphy, in Yorkshire (England) and Dotternhausen (Germany), that can be used to assess which model best explains the observed changes in carbon isotopes.
Abstract: Two hypotheses have been proposed to explain simultaneous large negative excursions (up to 7% PeeDee belemnite) in bulk carbonate (delta(13)C(carb)) and organic carbon isotope records (delta(13)C(org)) from black shales marking the Toarcian oceanic anoxic event (T-OAE). The first explanation envisions recycling of dissolved inorganic carbon (DIC) with a light isotopic signature into the photic zone from the lower levels of a salinity-stratified water mass, essentially requiring a regional paleoceanographic driver of the carbon cycle. The second involves the rapid and massive dissociation of methane from gas hydrates that effectively renders the T-OAE a global perturbation of the carbon cycle. We present C isotope records from belemnites (delta(13)C(bel)) sampled from two localities, calibrated with high-resolution ammonite biostratigraphy and Sr isotope stratigraphy, in Yorkshire (England) and Dotternhausen (Germany), that can be used to assess which model best explains the observed changes in carbon isotopes. Our records of the delta(13)C composition of belemnite calcite do not show the large negative C isotope excursions shown by coeval records of delta(13)C in sedimentary organic matter or bulk sedimentary carbonate. It follows that isotopically light carbon cannot have dominated the ocean-atmosphere carbon reservoir during the Toarcian OAE, as would be required were the methane release hypothesis correct. On the basis of an evaluation of available carbon isotope records we discuss a model in which the recycling of DIC from the deeper levels of a stratified water body, and shallowing of anoxic conditions into the photic zone, can explain all isotopic profiles. In particular, the model accounts for the higher C isotope values of belemnites that are characteristic of open ocean, well-mixed conditions, and the lower C isotope values of neritic phytoplankton communities that recorded the degree of density stratification and shallowing of anoxia in the photic zone.

190 citations


Journal ArticleDOI
TL;DR: In this paper, the first high-resolution alkenone-derived sea surface temperature (SST) reconstruction in the southeast Pacific (Ocean Drilling Program Site 1233) covering the major part of the last glacial period and the Holocene was presented.
Abstract: [1] We present the first high-resolution alkenone-derived sea surface temperature (SST) reconstruction in the southeast Pacific (Ocean Drilling Program Site 1233) covering the major part of the last glacial period and the Holocene. The record shows a clear millennial-scale pattern that is very similar to climate fluctuations observed in Antarctic ice cores, suggesting that the Southern Hemisphere high-latitude climate changes extended into the midlatitudes, involving simultaneous changes in air temperatures over Antarctica, sea ice extent, extension of the Antarctic Circumpolar Current, and westerly atmospheric circulation. A comparison to other midlatitude surface ocean records suggests that this “Antarctic” millennial-scale pattern was probably a hemisphere-wide phenomenon. In addition, we performed SST gradient reconstructions over the complete latitudinal range of the Pacific Eastern Boundary Current System for different time intervals during the last 70 kyr. The main results suggest an equatorward displaced subtropical gyre circulation during marine isotope stages 2 and 4.

189 citations


Journal ArticleDOI
TL;DR: In this article, the authors analyzed down-core samples of planktonic and benthic foraminifera for oxygen and carbon isotopes in order to study the interactions between climate change in the Northern Hemisphere and the western Mediterranean thermohaline circulation at times of Heinrich events (HE).
Abstract: Down-core samples of planktonic and benthic foraminifera were analyzed for oxygen and carbon isotopes in International Marine Past Global Changes Study (IMAGES) core MD99-2343 in order to study the interactions between climate change in the Northern Hemisphere and the western Mediterranean thermohaline circulation at times of Heinrich events (HE). Our results confirm the antiphase correlation between enhanced North Atlantic Deep Water formation and low ventilation in the Mediterranean. However, this study reveals that this antiphase relationship in deepwater formation between the North Atlantic and Mediterranean was interrupted during times of HE when the injection of large volumes of water from melting icebergs reached the entrance to the Mediterranean. These events, which lasted less than 1000 years, are represented by pronounced decreases in both planktonic d18O and benthic d13C signals. Lower salinities of Mediterranean surface water resulted in a slowdown of western Mediterranean deepwater overturn even though cold sea surface temperatures and drier climate should have resulted in enhanced deepwater formation.

180 citations



Journal ArticleDOI
TL;DR: In this article, the authors studied sediment core NP94-51 taken outside the mouth of the Hinlopen Strait on the northern Svalbard shelf and reconstructed the development of the last deglaciation and the Holocene using benthic and planktonic foraminifera, oxygen isotopes, and ice-rafted detritus.
Abstract: [1] The Svalbard archipelago is located in the high Arctic (76°–80°N) within the northernmost reach of the West Spitsbergen Current, which is the continuation of the North Atlantic Current. In this specific setting close to the Polar Front, even small variations in the current system are expected to give large and distinct signals in paleoceanographic parameters. Thus the Svalbard area is ideal for monitoring the past history of the inflow of Atlantic water to the Arctic Ocean. We have studied sediment core NP94-51 taken outside the mouth of the Hinlopen Strait on the northern Svalbard shelf. The paleoceanographic development of the last deglaciation and the Holocene has been reconstructed using benthic and planktonic foraminifera, oxygen isotopes, and ice-rafted detritus. The results show that the first strong subsurface inflow of Atlantic water to the Arctic Ocean after the Last Glacial Maximum commenced at 12.6 14C kyr B.P. (circa 15,000 calendar (cal) years B.P.) during the Bolling-Allerod interstadial. During the Younger Dryas, polar conditions prevailed at the surface with extensive sea ice cover, and the inflow of Atlantic-derived water was generally diminished. The inflow of more saline, but still cold Atlantic water was relatively strong during the early Holocene and caused intensified seasonal biological productivity. The Atlantic water inflow declined gradually during the mid-Holocene. Between 4500 and 1100 cal years B.P. the inflow was very weak, and the bottom waters over the northern Svalbard margin were cold and of lower salinity. During the last ∼1000 years, climatic conditions improved such that the subsurface inflow of Atlantic-derived water increased. However, the surface waters were still cold.

Journal ArticleDOI
TL;DR: In this article, the authors used diatom assemblage or physiological change on the diatom-bound 15N/14N is unknown and cannot yet be ruled out as a possible explanation for the observed change.
Abstract: [1] Diatom-bound 15N/14N was used to reconstruct the glacial nutrient status of the Subantarctic Zone in the Southern Ocean. Down-core records from both the Pacific and Indian sectors show δ15N of 5 to 6‰ during the Last Glacial Maximum and a decrease, coincident with the glacial termination, to values as low as 2‰. The effect of either diatom assemblage or physiological change on the diatom-bound 15N/14N is unknown and cannot yet be ruled out as a possible explanation for the observed change. However, the consistency between Indian and Pacific sector records and with other paleoceanographic data suggests that the glacial-interglacial difference in diatom-bound 15N/14N was driven by higher consumption of nitrate in the subantarctic surface during the last ice age. Such a change in nutrient consumption may have resulted from atmospheric iron fertilization and/or decreased glacial mixed layer depths associated with sea ice melting. Enhanced nutrient consumption in the glacial subantarctic would have worked to lower the concentration of CO2 in the ice age atmosphere. It also would have reduced the preformed nutrient content of the low-latitude thermocline, leading to decreases in low-latitude productivity, suboxia, and denitrification.

Journal ArticleDOI
TL;DR: A detailed study of the PETM stratigraphy from Ocean Drilling Program Site 690 (Weddell Sea) reveals that the CIE recovery period, which postdates the negative carbon isotope excursion onset by ∼80 kyr, is represented by an expanded (∼2.5 m thick) interval containing a unique planktic foraminiferal assemblage strongly diluted by coccolithophore carbonate as discussed by the authors.
Abstract: [1] The carbonate saturation profile of the oceans shoaled markedly during a transient global warming event known as the Paleocene-Eocene thermal maximum (PETM) (circa 55 Ma). The rapid release of large quantities of carbon into the ocean-atmosphere system is believed to have triggered this intense episode of dissolution along with a negative carbon isotope excursion (CIE). The brevity (120–220 kyr) of the PETM reflects the rapid enhancement of negative feedback mechanisms within Earth's exogenic carbon cycle that served the dual function of buffering ocean pH and reducing atmospheric greenhouse gas levels. Detailed study of the PETM stratigraphy from Ocean Drilling Program Site 690 (Weddell Sea) reveals that the CIE recovery period, which postdates the CIE onset by ∼80 kyr, is represented by an expanded (∼2.5 m thick) interval containing a unique planktic foraminiferal assemblage strongly diluted by coccolithophore carbonate. Collectively, the micropaleontological and sedimentological changes preserved within the CIE recovery interval reflect a transient state when ocean-atmosphere chemistry fostered prolific coccolithophore blooms that suppressed the local lysocline to relatively deeper depths. A prominent peak in the abundance of the clay mineral kaolinite is associated with the CIE recovery interval, indicating that continental weathering/runoff intensified at this time as well (Robert and Kennett, 1994). Such parallel stratigraphic changes are generally consonant with the hypothesis that enhanced continental weathering/runoff and carbonate precipitation helped sequester carbon during the PETM recovery period (e.g., Dickens et al., 1997; Zachos et al., 2005).

Journal ArticleDOI
TL;DR: In this paper, detailed deglacial and Holocene records of planktonic delta O-18 and Mg/Ca -based sea surface temperature (SST) from the Okinawa Trough suggest that at similar to 18 to 17 thousand years before present (kyr B. P), late spring/early summer SSTs were approximately 3 degrees C cooler than today, while surface waters were up to 1 practical salinity unit saltier.
Abstract: [1] Detailed deglacial and Holocene records of planktonic delta O-18 and Mg/Ca - based sea surface temperature (SST) from the Okinawa Trough suggest that at similar to 18 to 17 thousand years before present (kyr B. P.), late spring/ early summer SSTs were approximately 3 degrees C cooler than today, while surface waters were up to 1 practical salinity unit saltier. These conditions are consistent with a weaker influence of the summer East Asian Monsoon (EAM) than today. The timing of suborbital SST oscillations suggests a close link with abrupt changes in the EAM and North Atlantic climate. A tropical influence, however, may have resulted in subtle decoupling between the North Atlantic and the Okinawa Trough/EAM during the deglaciation. Okinawa Trough surface water trends in the Holocene are consistent with model simulations of an inland shift of intense EAM precipitation during the middle Holocene. Millennial-scale alternations between relatively warm, salty conditions and relatively cold, fresh conditions suggest varying influence of the Kuroshio during the Holocene.

Journal ArticleDOI
TL;DR: In this article, the major step in the middle Miocene global cooling (13.82 Ma ± 0.03) coincides with minimum eccentricity values associated with the 400-kyr cycle and minimum obliquity amplitudes associated with 1.2-Myr cycle.
Abstract: [1] Application of an astronomical age model to a bulk carbonate oxygen isotope record in the Ras il Pellegrin section on Malta indicates that the major step in the middle Miocene global cooling (13.82 Ma ± 0.03) coincides with minimum eccentricity values associated with the 400-kyr cycle and minimum obliquity amplitudes associated with the 1.2-Myr cycle. This orbital configuration is very similar to that found for comparable oxygen isotope enrichment events in the late Paleogene and Neogene. The stepwise character of the middle Miocene cooling event appears to be controlled by the combined influence of the 100-kyr eccentricity cycle and the 172-kyr cycle in obliquity amplitude. The integrated stratigraphy further allows extension of the astronomical polarity timescale to the top of chron C5ACn. The boundary between the Globigerina Limestone and the Blue Clay Formation coincides with the major step in middle Miocene global cooling and provides a level suitable for placing the physical reference point for the Langhian/Serravallian boundary.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the controls on organic carbon burial and molybdenum accumulation in ancient strata through an integration of biogeochemical modeling and data analysis.
Abstract: [1] In this study the controls on organic carbon burial and molybdenum accumulation in ancient strata are investigated through an integration of biogeochemical modeling and data analysis. Critical aspects of the study are employment of a biogeochemical model for organic matter degradation to explore the controls on pore water sulfide generation and authigenic molybdenum accumulation and use of accumulation rate data for Mo, OC, CaCO3, Fe, and Ti to reconstruct organic matter remineralization processes. The model results form a conceptual framework for the interpretation of primary production estimates and geochemical burial fluxes (calculated on the basis of a high-resolution orbital timescale) for the Cenomanian-Turonian (C-T) oceanic anoxic event 2 interval in the Western Interior basin. The results of this study suggest that the strong correlation between source rock development and intervals of transgression in the geologic record (such as the C-T) could reflect a confluence of biogeochemical processes within sediments that obviates the need for large-magnitude changes in primary production levels or oceanographic conditions (such as prolonged periods of stable water column stratification).

Journal ArticleDOI
TL;DR: In this article, Radiocarbon age relationships between co- occurring planktic foraminifera, alkenones, and total organic carbon in sediments from the continental margins of southern Chile, northwest Africa, and the South China Sea were compared with published results from the Namibian margin.
Abstract: [ 1] Radiocarbon age relationships between co- occurring planktic foraminifera, alkenones, and total organic carbon in sediments from the continental margins of southern Chile, northwest Africa, and the South China Sea were compared with published results from the Namibian margin. Age relationships between the sediment components are site- specific and relatively constant over time. Similar to the Namibian slope, where alkenones have been reported to be 1000 - 4500 years older than co- occurring foraminifera, alkenones were significantly ( similar to 1000 years) older than co- occurring foraminifera in the Chilean margin sediments. In contrast, alkenones and foraminifera were of similar age ( within 2 sigma error or better) in the NW African and South China Sea sediments. Total organic matter and alkenone ages were similar off Namibia ( age difference TOC alkenones: 200 - 700 years), Chile ( 100 - 450 years), and NW Africa ( 360 - 770 years), suggesting minor contributions of preaged terrigenous material. In the South China Sea, total organic carbon is significantly ( 2000 - 3000 years) older owing to greater inputs of preaged terrigenous material. Age offsets between alkenones and planktic foraminifera are attributed to lateral advection of organic matter. Physical characteristics of the depositional setting, such as seafloor morphology, shelf width, and sediment composition, may control the age of co- occurring sediment components. In particular, offsets between alkenones and foraminifera appear to be greatest in deposition centers in morphologic depressions. Aging of organic matter is promoted by transport. Age offsets are correlated with organic richness, suggesting that formation of organic aggregates is a key process.

Journal ArticleDOI
TL;DR: In this article, a coupled calcium/carbon cycle model was proposed to explain a large portion of the observed δ44/40Casw variations in planktonic foraminifera isotopic composition.
Abstract: Measurements of the calcium isotopic composition (δ44/40Ca) of planktonic foraminifera from the western equatorial Pacific and the Indian sector of the Southern Ocean show variations of about 0.6‰ over the past 24 Myr. The stacked δ44/40Ca record of Globigerinoides trilobus and Globigerina bulloides indicates a minimum in δ44/40Casw (seawater calcium) at 15 to 16 Ma and a subsequent general increase toward the present, interrupted by a second minimum at 3 to 5 Ma. Applying a coupled calcium/carbon cycle model, we find two scenarios that can explain a large portion of the observed δ44/40Casw variations. In both cases, variations in the Ca input flux to the ocean without proportional changes in the carbonate flux are invoked. The first scenario increases the riverine calcium input to the ocean without a proportional increase of the carbonate flux. The second scenario generates an additional calcium flux from the exchange of Ca by Mg during dolomitization. In both cases the calcium flux variations lead to drastic changes in the seawater Ca concentrations on million year timescales. Our δ44/40Casw record therefore indicates that the global calcium cycle may be much more dynamic than previously assumed.

Journal ArticleDOI
TL;DR: In this paper, the authors applied stable isotope, trace metal, alkenone paleothermometry, and radiocarbon methods to sediment cores from the Laurentian Fan to reveal the history of climate in that region over the past ∼11 kyr.
Abstract: [1] Stable isotope, trace metal, alkenone paleothermometry, and radiocarbon methods have been applied to sediment cores in the western subpolar North Atlantic between Hudson Strait and Cape Hatteras to reveal the history of climate in that region over the past ∼11 kyr. We focus on cores from the Laurentian Fan, which is known to have rapid and continuous accumulation of hemipelagic sediment. Although results among our various proxy data are not always in agreement, the weight of the evidence (alkenone sea surface temperature (SST), δ18O and abundance of Globigerinoides ruber) indicates a continual cooling of surface waters over Laurentian Fan, from about 18°C in the early Holocene to about 8°C today. Alternatively, Mg/Ca data on planktonic foraminifera indicate no systematic change in Holocene SST. The inferred long-term decrease in SST was probably driven by decreasing seasonality of Northern Hemisphere insolation. Two series of proxy data show the gradual cooling was interrupted by a two-step cold pulse that began 8500 years ago, and lasted about 700 years. Although this event is associated with the final deglaciation of Hudson Bay, there is no δ18O minimum anywhere in the Labrador Sea, yet there is some evidence for it as far south as Cape Hatteras. Finally, although the 8200 year B.P. event has been implicated in decreasing North Atlantic ventilation, and hence widespread temperature depression on land and at sea, we find inconsistent evidence for a change at that time in deep ocean nutrient content at ∼4 km water depth.

Journal ArticleDOI
TL;DR: In this article, Dinocysts from cores collected in the Chukchi Sea from the shelf edge to the lower slope were used to reconstruct changes in sea surface conditions and sea ice cover using modern analogue techniques.
Abstract: [1] Dinocysts from cores collected in the Chukchi Sea from the shelf edge to the lower slope were used to reconstruct changes in sea surface conditions and sea ice cover using modern analogue techniques. Holocene sequences have been recovered in a down-slope core (B15: 2135 m, 75°44′N, sedimentation rate of ∼1 cm kyr−1) and in a shelf core (P1: 201 m, 73°41′N, sedimentation rate of ∼22 cm kyr−1). The shelf record spanning about 8000 years suggests high-frequency centennial oscillations of sea surface conditions and a significant reduction of the sea ice at circa 6000 and 2500 calendar (cal) years B.P. The condensed offshore record (B15) reveals an early postglacial optimum with minimum sea ice cover prior to 12,000 cal years B.P., which corresponds to a terrestrial climate optimum in Bering Sea area. Dinocyst data indicate extensive sea ice cover (>10 months yr−1) from 12,000 to 6000 cal years B.P. followed by a general trend of decreasing sea ice and increasing sea surface salinity conditions, superimposed on large-amplitude millennial-scale oscillations. In contrast, δ18O data in mesopelagic foraminifers (Neogloboquadrina pachyderma) and benthic foraminifers (Cibicides wuellerstorfi) reveal maximum subsurface temperature and thus maximum inflow of the North Atlantic water around 8000 cal years B.P., followed by a trend toward cooling of the subsurface to bottom water masses. Sea-surface to subsurface conditions estimated from dinocysts and δ18O data in foraminifers thus suggest a decoupling between the surface water layer and the intermediate North Atlantic water mass with the existence of a sharp halocline and a reverse thermocline, especially before 6000 years B.P. The overall data and sea ice reconstructions from core B15 are consistent with strong sea ice convergence in the western Arctic during the early Holocene as suggested on the basis of climate model experiments including sea ice dynamics, matching a higher inflow rate of North Atlantic Water.

Journal ArticleDOI
TL;DR: The Toarcian oceanic anoxic event (∼183 Myr ago) represents a global perturbation marked by increasing organic carbon burial and a general decrease in calcium carbonate production likely triggered by elevated carbon dioxide levels in the atmosphere as mentioned in this paper.
Abstract: [1] The Toarcian oceanic anoxic event (∼183 Myr ago) represents a global perturbation marked by increasing organic carbon burial and a general decrease in calcium carbonate production likely triggered by elevated carbon dioxide levels in the atmosphere. Here we present quantitative analyses of calcareous nannofossil diversity and abundance from the Castillo de Pedroso section in Cantabria, northern Spain. We compare these data with geochemical data (C and O isotopes) obtained from biogenic and bulk carbonate records in order to highlight the response of calcareous phytoplankton to major climatic and paleoceanographic changes. The Pliensbachian/Toarcian boundary is characterized by an abrupt decrease in abundance of Schizosphaerella punctulata, the most important lithogenic contributor to (hemi) pelagic carbonates in the Early Jurassic. The early Toarcian nannofloral assemblages show an increase in abundance of Mitrolithus jansae and small-sized r-selected taxa and a progressive decrease in S. punctulata percentages. The deep dwellers M. jansae and S. punctulata experienced a major crisis slightly prior to the deposition of the Toarcian black shales that are characterized by high abundances of eutrophic taxa such as Lotharingius spp. and Biscutum spp. The return of S. punctulata associated with lower percentages of eutrophic taxa was observed just above the Toarcian black shales. The Toarcian episode reveals that high CO2 levels and increasing primary productivity probably triggered a shift in abundance from highly calcified nannoliths such as S. punctulata and M. jansae to small-sized r-selected coccoliths that overall record a biocalcification crisis at the onset and during the Toarcian episode.

Journal ArticleDOI
TL;DR: This paper measured the Mg/Ca ratio in two surface-dwelling species of planktonic foraminifera, Globigerinoides ruber and Globigerina bulloides, from biweekly sediment trap samples collected in Guaymas Basin, Gulf of California, between August 1992 and November 1997.
Abstract: [1] The Mg/Ca ratio has been measured in two surface-dwelling species of planktonic foraminifera, Globigerinoides ruber and Globigerina bulloides, from biweekly sediment trap samples collected in Guaymas Basin, Gulf of California, between August 1992 and November 1997. The Guaymas Basin experiences significant seasonal changes in sea surface temperature (SST) (∼16°–33°C) and thus is an ideal location for testing the temperature dependence of Mg incorporation into foraminiferal calcite. The planktonic foraminiferal Mg/Ca ratios are directly compared with concurrent temperature measurements from the study site. The results from the sediment trap study reveal a strong positive correlation between SST and the Mg/Ca ratio in both G. ruber (r2 = 0.86) and G. bulloides (r2 = 0.90). The Mg/Ca ratio increases exponentially by 7% per 1°C change in temperature for G. ruber and by 6% for G. bulloides. These results indicate that the Mg/Ca ratio in G. ruber and G. bulloides accurately records the measured seasonal surface temperature cycle and interannual variability in the Guaymas Basin within ±1°C. The greatest deviation between Mg/Ca–estimated SST and observed SST occurs in the spring and fall for both species. This variability is attributed to rapidly changing hydrographic conditions and possible offset between the time of calcification and the temperature measurements. In addition, G. ruber and G. bulloides exhibit very high Mg/Ca ratios during the strong El Nino–Southern Oscillation years of 1992 and 1997, verifying their potential to record SST up to 33°C and 28°C, respectively. The trace metal data are compared with foraminiferal oxygen isotope and alkenone measurements made on the same suite of sediment trap samples to assess the internal consistency of these temperature proxies. Strong correlations exist between the δ18O and Mg/Ca for G. ruber (r2 = 0.86) and G. bulloides (r2 = 0.79). Similarly, sea surface temperature estimates from Mg/Ca ratios are in good agreement with alkenone-derived temperature estimates (r2 = 0.89 for G. ruber and r2 = 0.81 for G. bulloides).

Journal ArticleDOI
TL;DR: The Eocene compensation depth (CCD) was relatively shallow near the early Eocene Pacific equator, 3200 m, and unlike modern latitudinal CCD gradients deepened to the north (to ∼3600 m; paleolatitude ∼10°N) at 41 Ma as discussed by the authors.
Abstract: [1] Ocean Drilling Program Leg 199 drilled a north-south transect across the Eocene paleoequator in the eastern Pacific, permitting reconstruction the calcite compensation depth (CCD) since earliest Eocene time. The CCD was relatively shallow near the early Eocene Pacific equator, 3200 m, and unlike modern latitudinal CCD gradients deepened to the north (to ∼3600 m; paleolatitude ∼10°N). At 41 Ma the CCD underwent a brief, sharp, transient deepening of 700 m, then remained shallow until the Eocene/Oligocene boundary. At the E/O boundary, the CCD deepened by 1200 m in less than 300 kyr. This rapid deepening served to more than double the area of seafloor subject to CaCO3 deposition. Sea level fall associated with ice volume buildup, and ensuing shelf-basin fractionation, is unlikely to be the sole cause of the increased deep-ocean CaCO3 burial; rather, a sudden, rapid increase in the amount of Ca entering the ocean appears necessary to explain the observations.

Journal ArticleDOI
TL;DR: In this article, a growth-dependent Strontium to calcium ratio (Sr/Ca) calibration equation was derived to reconstruct western subtropical North Atlantic sea surface temperature (SST) for the past 223 years.
Abstract: [1] Strontium to calcium ratios (Sr/Ca) are reported for a massive brain coral Diploria labyrinthiformis collected from the south shore of Bermuda and are strongly correlated with both sea surface temperature (SST) and mean annual skeletal growth rate. High Sr/Ca ratios correspond with cold SSTs and slow skeletal growth rate and vice versa. We provide a quantitative calibration of Sr/Ca to extension rate and SST along the axis of maximum growth and derive a growth-dependent Sr/Ca–SST calibration equation to reconstruct western subtropical North Atlantic SSTs for the past 223 years. When the influence of growth rate is excluded from the calibration, Sr/Ca ratios yield SSTs that are too cold during cool anomalies and too warm during warm anomalies. Toward the end of the Little Ice Age (∼1850), SST changes derived using a calibration that is not growth-dependent are exaggerated by a factor of 2 relative to those from the growth-corrected model that yields SSTs ∼1.5°C cooler than today. Our results indicate that incorporation of growth rate effects into coral Sr/Ca calibrations may improve the accuracy of SSTs derived from living and fossil corals.

Journal ArticleDOI
TL;DR: Foraminifer, diatom, and ostracod assemblages from ocean cores reveal a warm phase of the Pliocene between about 3.3 and 3.0 Ma as mentioned in this paper.
Abstract: [1] Estimates of sea surface temperature (SST) based upon foraminifer, diatom, and ostracod assemblages from ocean cores reveal a warm phase of the Pliocene between about 3.3 and 3.0 Ma. Pollen records and plant megafossils, although not as well dated, show evidence for a warmer climate at about the same time. Increased greenhouse forcing and altered ocean heat transport are the leading candidates for the underlying cause of Pliocene global warmth. Despite being a period of global warmth, this interval encompasses considerable variability. Two new SST reconstructions are presented that are designed to provide a climatological error bar for warm peak phases of the Pliocene and to document the spatial distribution and magnitude of SST variability within the mid-Pliocene warm period. These data suggest long-term stability of low-latitude SST and document greater variability in regions of maximum warming.

Journal ArticleDOI
TL;DR: In this article, Mg/Ca analyses of G. bulloides and abundances of N. pachyderma from Ocean Drilling Program (ODP) Leg 175 Hole 1084B in the Benguela coastal upwelling system document lower sea surface temperatures during the Last Glacial Maximum (LGM), Younger Dryas, mid-Holocene, and Little Ice Age in the southeastern Atlantic.
Abstract: [1] Mg/Ca analyses of G. bulloides and abundances of N. pachyderma (left coiling) from Ocean Drilling Program (ODP) Leg 175 Hole 1084B in the Benguela coastal upwelling system document lower sea surface temperatures during the Last Glacial Maximum (LGM), Younger Dryas, mid-Holocene, and Little Ice Age in the southeastern Atlantic. Taking into consideration the possible effects of differential carbonate dissolution, the Mg/Ca data indicate Younger Dryas temperatures 2°–3°C cooler than those of the early Holocene and LGM temperatures 4°–5°C cooler than those of the early Holocene. The cool interval during the deglacial period at Hole 1084B matches the timing of Younger Dryas shifts in Cariaco Basin and Greenland Ice Sheet records and that of a nearby alkenone record. Comparison of mid-Holocene cooling at Hole ODP1084B with other high-resolution records of Holocene and last deglacial sea surface temperatures from the tropical Atlantic implies consistent basin-wide changes in atmospheric circulation. A brief period of 1.5°–2°C cooling between 17.8 and 17.2 ka, if related to Heinrich event 1, is consistent with a previously hypothesized tropical origin of all Heinrich climate change events.

Journal ArticleDOI
TL;DR: In this article, the authors find no evidence of this level of sediment focusing in the equatorial Pacific from geophysical data or sediment core comparisons and conclude that the overestimation of the sediment focusing by 230Th is probably caused by poor model assumptions: that sediment does not fractionate (does not sort according to size during transport) and that 230Th cannot leak from slowly accumulating sediments.
Abstract: [1] The paleoceanographic recording fidelity of pelagic sediments is limited by chemical diagenesis and physical mixing (bioturbation and horizontal sediment transport) Diagenesis and bioturbation are relatively well-studied, but the effects of physical sedimentation have been largely ignored Modeling U series isotopes (eg, 230Th) can potentially quantify horizontal sediment movement, but model horizontal sediment focusing often equals or exceeds the vertical particle rain We find no evidence of this level of sediment focusing in the equatorial Pacific from geophysical data or sediment core comparisons The overestimate of sediment focusing by 230Th is probably caused by poor model assumptions: that sediment does not fractionate (does not sort according to size during transport) and that 230Th cannot leak from slowly accumulating sediments Both assumptions are weak U series methods do hold promise to quantify sediment focusing if properly calibrated With calibration the trade-offs between seeking high sedimentation rates for better time resolution and the blurring by horizontal sediment focusing can be better assessed

Journal ArticleDOI
TL;DR: In this paper, the authors used filtered particulate organic matter (POM) samples from the surface waters of the Nordic seas to monitor the spatial distribution of long-chain alkenones.
Abstract: [1] Samples of filtered particulate organic matter (POM) were obtained during the summers of 1999 and 2000 from the surface waters of the Nordic seas to monitor the spatial distribution of long-chain alkenones. The aim of the study was to appraise existing alkenone-based climatic proxies in northern high latitudes. Unusually high percentages of the tetraunsaturated alkenone were measured in the polar waters of the East Greenland Current, with C37:4 of up to 77% in 80% of sea-ice cover. Values of percent C37:4 across the Nordic seas showed a strong association with water mass type. Analysis of coccoliths in filters indicated that calcified Emiliania huxleyi could not be discounted as the biological precursor of alkenones in all the water masses. A combined data set of 69 samples of POM revealed a stronger correlation of percent C37:4 to sea surface salinity (SSS; R2 = 0.72) than to sea surface temperature (SST; R2 = 0.50). Values of percent C37:4 in sea surface POM were much higher than those in surficial sediments of the northern North Atlantic. To explain the discrepancy in sedimentary and surface water column percent C37:4, we propose that the alkenone contents in surface sediments underlying arctic and polar waters are a combination of autochthonous and allochthonous inputs of alkenones. Our results show that percent C37:4 can be used to reconstruct the relative extension of arctic/polar water masses in the North Atlantic. However, the results prevent confirmation of percent C37:4 as a paleo-SSS proxy in the Nordic seas, given its multivariate nature in our data set and the decoupling between its range of values in surface waters and sediments.

Journal ArticleDOI
TL;DR: In this paper, the authors present micropalaeontological and grain-size records for a set of sediment cores from the Gulf of Cadiz (southwest Spain) that reflect changes in the position and strength of the Mediterranean Outflow (MO) current.
Abstract: We present micropalaeontological and grain-size records for a set of sediment cores from the Gulf of Cadiz (southwest Spain) that reflect changes in the position and strength of the Mediterranean Outflow (MO) current. The cores sample a sediment drift (the Gil Eanes Drift) that is positioned lower on the slope in the Gulf of Cadiz than the position of the main current today. The data indicate that the drift is of glacial age and that the glacial MO current was positioned lower on the slope than today but also that it was active over a considerably reduced area of the slope. We argue that this observation is consistent with physical constraints on the Gibraltar Exchange and on the likely settling and spreading behavior of the MO plume along the Iberian Margin under glacial environmental and sea level conditions. The deeper settling of the MO is likely to have influenced the formation of glacial North Atlantic Intermediate Water and also may have exerted indirect influence on the formation of glacial North Atlantic Deep Water.