Radiolarian chronology and fauna across the middle/late Eocene boundary at ODP Hole 171-1052A

Quantitative radiolarian assemblage analysis has been conducted on middle and upper Eocene sediments (Zones RP16 to RP18) from Ocean Drilling Program Site 1052 in order to establish the radiolarian magnetobiochronology and determine the nature of the faunal turnover across the middle/late Eocene boundary in the western North Atlantic Ocean. We recognize and calibrate forty-five radiolarian bioevents to the magneto- and cyclo-stratigraphy from Site 1052 to enhance the biochronologic resolution for the middle and late Eocene. Our data is compared to sites in the equatorial Pacific (Leg 199) to access the diachrony of biostratigraphic events. Eleven bioevents are good biostratigraphic markers for tropical/subtropical locations (south of 30°N). The primary markers (lowest occurrences of Cryptocarpium azyx and Calocyclas bandyca) which are tropical zonal boundary markers for Zones RP17 and RP18 provide robust biohorizons for correlation and age determination from the low to middle latitudes and between the Atlantic and Pacific Oceans. Some other radiolarian bioevents are highly diachronous (<1 million years) between oceanic basins. A significant faunal turnover of radiolarians is recognized within Chron C17n.3n (37.7 Ma) where 13 radiolarian species disappear rapidly in less than 100 kyr and 4 new species originate. The radiolarian faunal turnover coincides with a major extinction in planktonic foraminifera. We name the turnover phase, the Middle/Late Eocene Turnover (MLET). Assemblage analysis reveals the MLET to be associated with a decrease in low-mid latitude taxa and increase in cosmopolitan taxa and radiolarian accumulation rates. The MLET might be related to increased biological productivity rather than to surface-water cooling.

Radiocarbon ages and plateau definitions of sediments from the abyssal ocean

Ice core records demonstrate a glacial-interglacial atmospheric CO2 increase by ~100 ppm, while 14C calibration efforts document a strong decrease in atmospheric 14C concentration during this period. A calculated transfer of ~530 Gt of 14C depleted carbon is required to produce the deglacial coeval rise of carbon in the atmosphere and terrestrial biosphere. This amount is usually ascribed to oceanic carbon release, although the actual mechanisms remained elusive, since an adequately old and carbon-enriched deep-ocean reservoir seemed unlikely. Here we present a new, though still fragmentary, ocean-wide d14C dataset showing that during the Last Glacial Maximum (LGM) and Heinrich Stadial 1 (HS-1) the maximum 14C age difference between ocean deep waters and the atmosphere exceeded the modern values by up to 1500 14C yr, in the extreme reaching 5100 14C yr. Below 2000 m depth the 14C ventilation age of modern ocean waters is directly linked to the concentration of dissolved inorganic carbon (DIC). We propose as working hypothesis that the modern regression of DIC vs d14C also applies for LGM times, which implies that a mean LGM aging by ~600 14C yr corresponded to a global rise of ~85-115 µmol DIC/kg in the deep ocean. Thus, the prolonged residence time of ocean deep waters may indeed have made it possible to absorb an additional ~730-980 Gt DIC, one third of which possibly originated from intermediate waters. We also infer that LGM deep-water O2 dropped to suboxic values of <10µmol/kg in the Atlantic sector of the Southern Ocean, possibly also in the subpolar North Pacific. The outlined deglacial transfer of the extra aged, deep-ocean carbon to the atmosphere via the dynamic ocean-atmosphere carbon exchange would be sufficient to account for two trends observed, (1) for the increase in atmospheric CO2 and (2) for the 190-permil drop in atmospheric d14C during the so-called HS-1 'Mystery Interval', when atmospheric 14C production rates were largely constant.

(Table 1) Gas tie points used to correlate NorthGRIP and Vostol records

North Atlantic sediment records (MD95-2042), Greenland (Greenland Ice Core Project (GRIP)) and Antarctica (Byrd and Vostok) ice core climate records have been synchronized over marine isotopic stage 3 (MIS 3) (64 to 24 kyr B.P.) (Shackleton et al., 2000). The resulting common timescale suggested that MD95-2042 d18Obenthic fluctuations were synchronous with temperature changes in Antarctica (dDice or d18Oice records). In order to assess the persistency of this result we have used here the recent Greenland NorthGRIP ice core covering the last glacial inception. We transfer the Antarctic Vostok GT4 timescale to NorthGRIP d18Oice and MD95-2042 d18Oplanktonic records and precisely quantify all the relative timing uncertainties. During the rapid warming of Dansgaard-Oeschger 24, MD95-2042 d18Obenthic decrease is in phase with d18Oplanktonic decrease and therefore with NorthGRIP temperature increase, but it takes place 1700 ± 1100 years after the Antarctic warming. Thus the present study reveals that the results obtained previously for MIS 3 cannot be generalized and demonstrates the need to improve common chronologies for marine and polar archives.

Geochemistry of foraminifera from sediment cores of the central California margin

Under present climate conditions, convection at high latitudes of the North Pacific is restricted to shallower depths than in the North Atlantic. To what extent this asymmetry between the two ocean basins was maintained over the past 20 kyr is poorly known because there are few unambiguous proxy records of ventilation from the North Pacific. We present new data for two sediment cores from the California margin at 800 and 1600 m depth to argue that the depth of ventilation shifted repeatedly in the northeast Pacific over the course of deglaciation. The evidence includes benthic foraminiferal Cd/Ca, 18O/16O, and 13C/12C data as well as radiocarbon age differences between benthic and planktonic foraminifera. A number of features in the shallower of the two cores, including an interval of laminated sediments, are consistent with changes in ventilation over the past 20 kyr suggested by alternations between laminated and bioturbated sediments in the Santa Barbara Basin and the Gulf of California [Keigwin and Jones, 1990 doi:10.1029/PA005i006p01009; Kennett and Ingram, 1995 doi:10.1038/377510a0; Behl and Kennett, 1996 doi:10.1038/379243a0]. Data from the deeper of the two California margin cores suggest that during times of reduced ventilation at 800 m, ventilation was enhanced at 1600 m depth, and vice versa. This pronounced depth dependence of ventilation needs to be taken into account when exploring potential teleconnections between the North Pacific and the North Atlantic.

Nitrogen isotopic composition in South China Sea sediments

The d15N of surface and down-core sediments spanning the last 20-200 kyr from the entire South China Sea (SCS) ranges only from ~3.0 to ~6.5 per mil, with no correlation with discernible paleoclimatic/oceanographic changes. Detailed profiles of the uppermost sediment column, including fluff samples, indicate a minor diagenetic overprint of 0.3-1.2 per mil at the sediment-water interface. The absence of any correlation with reconstructed (glacial-interglacial) changes in primary production, terrigenous input, and/or sea level related basin configuration is attributed to a complete consumption of nitrate during primary production in this marginal basin during at least the last 140,000 years. This, in turn, implies that the d15N of the nitrate used during primary production remained approximately constant during the last climatic cycle. The proposed scenario infers an unchanged nitrogen isotopic composition of the western Pacific subsurface nitrate between glacial and interglacial stages as well as during terminations and thus constrains proposed changes in the oceanic N inventory.

Age determination of sediment core MD99-2201

A marine sediment core from the leeward margin of Great Bahama Bank (GBB) was subjected to a multiproxy study. The aragonite dominated core MD992201 comprises the past 7230 years in a decadal time resolution and shows sedimentation rates of up to 13.8 m/kyr. Aragonite mass accumulation rates, age differences between planktonic foraminifera and aragonite sediments, and temperature distribution are used to deduce changes in aragonite production rates and paleocurrent strengths. Aragonite precipitation rates on GBB are controlled by exchange of carbonate ions and CO2 loss due to temperature-salinity conditions and biological activity, and these are dependent on the current strength. Paleocurrent strengths on GBB show high current velocities during the periods 6000-5100 years BP, 3500-2700 years BP, and 1600-700 years BP; lower current speeds existed during the time intervals 5100-3500 years BP, 2700-1600 years BP, and 700-100 years BP. Bahamian surface currents are directly linked to the North Atlantic atmospheric circulation, and thus periods with high (low) current speeds are proposed to be phases of strong (weak) atmospheric circulation.

Temperature proxies of sediment cores from the Sea of Marmara

Reconstructing ocean temperature values is a major target in paleoceanography and climate research. However, most temperature proxies are organism-based and thus suffer from an "ecological bias". Multiproxy approaches can potentially overcome this bias but typically require more investment in time and resources, while being susceptible to errors induced by sample preparation steps necessary before analysis. Three lipid-based temperature proxies are widely used: UK'37 (based on long chain alkenones from phytoplanktonic haptophytes), TEX86 [based on glycerol dialkyl glycerol tetraethers (GDGTs) from pelagic archaea] and LDI (based on long chain diols from phytoplanktonic eustigmatophytes). So far, separate analytical methods, including gas chromatography (GC) and liquid chromatography (LC), have been used to determine these proxies. Here we present a sensitive method for determining all three in a single normal phase high performance LC-atmospheric pressure chemical ionization mass spectrometry (NP-HPLC-APCI-MS) analysis. Each of the long chain alkenones and long chain diols was separated and unambiguously identified from the accurate masses and characteristic fragmentation during multiple stage MS analysis (MS2). Comparison of conventional GC and HPLC-MS methods showed similar results for UK'37 and LDI, respectively, using diverse environmental samples and an Emiliania huxleyi culture. Including the three sea surface temperature (SST) proxies; the NP-HPLC-APCI-MS method in fact allows simultaneous determination of nine paleoenvironmental proxies. The extent to which the ecology of the source organisms (ecological bias) influences lipid composition and thereby the reconstructed temperature values was demonstrated by applying the new method to a sediment core from the Sea of Marmara, covering the last 21 kyr BP. Reconstructed SST values differed considerably between the proxies for the Last Glacial Maximum (LGM) and the period of Sapropel S1 formation at ca. 10 kyr BP, whereas the trends during the late Holocene were similar. Changes in the composition of alkenone-producing species at the transition from the LGM to the Bølling/Allerød (B/A) were inferred from unreasonably high UK'37-derived SST values (ca. 20 °C) during the LGM. We ascribe discrepancies between the reconstructed temperature records during S1 deposition to habitat change, e.g. a different depth due to changes in nutrient availability.

Palynology on sediment profile Tso Kar in Ladakh

Palynological investigation of a 410 cm long core section from Tso Kar (33°10'N, 78°E, 4527 m a.s.l.), an alpine lake situated in the arid Ladakh area of NW India at the limit of the present-day Indian summer monsoon, was performed in order to reconstruct post-glacial regional vegetation and climate dynamics. The area was covered with alpine desert vegetation from ca. 15.2 to 14 kyr BP (1 kyr=1000 cal. years), reflecting dry and cold conditions. High influx values of long-distance transported Pinus sylvestris type pollen suggest prevailing air flow from the west and northwest. The spread of alpine meadow communities and local aquatic vegetation is a weak sign of climate amelioration after ca. 14 kyr BP. Pollen data (e.g. influx values of Pinus roxburghii type and Quercus) suggest that this was due to a strengthening of the summer monsoon and the reduced activity of westerly winds. The further spread of Artemisia and species-rich meadows occurred in response to improved moisture conditions between ca. 12.9 and 12.5 kyr BP. The subsequent change towards drier desert-steppe vegetation likely indicates more frequent westerly disturbances and associated snowfalls, which favoured the persistence of alpine meadows on edaphically moist sites. The spread of Chenopodiaceae-dominated vegetation associated with an extremely weak monsoon occurred at ca. 12.2-11.8 kyr BP during the Younger Dryas interstadial. A major increase in humidity is inferred from the development of Artemisia-dominated steppe and wet alpine meadows with Gentianaceae after the late glacial/early Holocene transition in response to the strengthening of the summer monsoon. Monsoonal influence reached maximum activity in the Tso Kar region between ca. 10.9 and 9.2 kyr BP. The subsequent development of the alpine meadow, steppe and desert-steppe vegetation points to a moderate reduction in the moisture supply, which can be linked to the weaker summer monsoon and the accompanying enhancement of the winter westerly flow from ca. 9.2 to 4.8 kyr BP. The highest water levels of Tso Kar around 8 kyr BP probably reflect combined effect of both monsoonal and westerly influence in the region. An abrupt shift towards aridity in the Tso Kar region occurred after ca. 4.8 kyr BP, as evidenced by an expansion of Chenopodiaceae-dominated desert-steppe. Low pollen influx values registered ca. 2.8-1.3 kyr BP suggest scarce vegetation cover and unfavourable growing conditions likely associated with a further weakening of the Indian Monsoon.

(Table 1) Gas tie points used to correlate the ice cores NGRIP and EDML

Water isotope records from the EPICA Dronning Maud Land (EDML) and the NorthGRIP ice cores have revealed a one to one coupling between Antarctic Isotope Maxima (AIM) and Greenland Dansgaard-Oeschger (DO) events back to 50 kyr. In order to explore if this north-south coupling is persistent over Marine Isotopic Stage 5 (MIS 5), a common timescale must first be constructed. Here, we present new records of d18O of O2 (d18Oatm) and methane (CH4) measured in the air trapped in ice from the EDML (68-147 kyr) and NorthGRIP (70-123 kyr) ice cores. We demonstrate that, through the period of interest, CH4 records alone are not sufficient to construct a common gas timescale between the two cores. Millennial-scale variations of d18Oatm are evidenced over MIS 5 both on the Antarctic and Greenland ice cores and are coupled to CH4 profiles to synchronise the NorthGRIP and EDML records. They are shown to be a precious tool for ice core synchronisation. With this new dating strategy, we produce the first continuous and accurate sequence of the north-south climatic dynamics on a common ice timescale for the last glacial inception and the first DO events of MIS 5, reducing relative dating uncertainties to an accuracy of a few centuries at the onset of DO events 24 to 20. This EDML-NorthGRIP synchronisation provides new firm evidence that the bipolar seesaw is a pervasive pattern from the beginning of the glacial period. The relationship between Antarctic warming amplitudes and their concurrent Greenland stadial duration highlights the particularity of DO event 21 and its Antarctic counterpart. Our results suggest a smaller Southern Ocean warming rate for this long DO event compared to DO events of MIS 3.

Mid-Pleistocene planktonic foraminiferal composition and IRD content in ODP Sites 162-980 and 162-984

We investigated surface and deep ocean variability in the subpolar North Atlantic from 1000 to 500 thousand years ago (ka) based on two Ocean Drilling Program (ODP) sites, Feni drift site 980 (55°29'N, 14°42'W) and Bjorn drift site 984 (61°25'N, 24°04'W). Benthic foraminiferal stable isotope data, planktic foraminiferal faunas, ice-rafted debris data, and faunally based sea-surface temperature estimates help test the hypothesis that oceanographic changes in the North Atlantic region were associated with the onset of the 100-kyr world during the mid-Pleistocene revolution. Based on percentage of Neogloboquadrina pachyderma (s) records from both sites, surface waters during interglacials and glacials were cooler in the mid-Pleistocene than during marine isotope stages (MIS) 5 and 6. In particular, interglaciations at Bjorn drift site 984 were significantly cooler. Faunal evidence suggests that the interglacial Arctic front shifted from a position between the two sites to a position northwest of Bjorn drift site 984 after ca. 610 ka. As during the late Pleistocene, we find faunal evidence for lagging surface warmth at most of the glacial initiations during the mid-Pleistocene. Each initiation is associated with high benthic d13C values that are maintained into the succeeding glaciation, which we term "lagging NADW production." These findings indicate that lagging warmth and lagging NADW production are robust features of the regional climate system that persist in the middle to late Pleistocene.

Ice rafted debris in the Nordic Seas

A continuous 3.5 Myr IRD record was produced from Ocean Drilling Program (ODP) Site 907. A timescale based on magnetic polarity chrons, oxygen isotope stratigraphy (for the last 1Myr) and orbital tuning was developed. The record documents a stepwise inception of large-scale glacial cycles in the Nordic Seas region, the first being a marked expansion of the Greenland ice sheet at 3.3 Ma. A second step occurred at 2.74 Ma by an expansion of large scale ice sheets in the Northern Hemisphere. Ice sheet variability around the Nordic Seas was tightly coupled to global ice volume over the past 3.3 Myr. Between 3 and 1 Ma, most of the variance of the IRD signal is in the 41 kyr band, whereas the last 1 Myr is characterized by stronger 100 kyr variance. The Gamma Ray Porosity Evaluator (GRAPE) density record is closely linked with IRD variations and documents sub orbital variability resembling the late Quaternary Heinrich/Bond cycles.

Sapropel cycles in the Mediterranean Sea

Water column stratification increased at climatic transitions from cold to warm periods during the late Quaternary and led to anoxic conditions and sapropel formation in the deep eastern Mediterranean basins. High-resolution data sets on sea-surface temperatures (SST) (estimated from UK'37 indices) and d18O of planktonic foraminifer calcite (d18Ofc) across late Pleistocene sapropel intervals show that d18Ofc decreased (between 1 and 4.6 per mil) and SST increased (between 0.7° and 6.7°C). Maximal d18Oseawater depletion of eastern Mediterranean surface waters at the transition is between 0.5 and 3.0 per mil, and in all but one case exceeded the depletion seen in a western Mediterranean core. The depletion in d18Oseawater is most pronounced at sapropel bases, in agreement with an initial sudden input of monsoon-derived freshwater. Most sapropels coincide with warming trends of SST. The density decrease by initial freshwater input and continued warming of the sea surface pooled fresh water in the surface layer and prohibited deep convection down to ageing deep water emplaced during cold and arid glacial conditions. An exception to this pattern is "glacial" sapropel S6; its largest d18Oseawater depletion (3 per mil) is almost matched by the depletion in the western Mediterranean Sea, and it is accompanied by surface water cooling following an initially rapid warming phase. A second period of significant isotopic depletion is in isotope stage 6 at the 150 kyr insolation maximum. While not expressed as a sapropel due to cold SST, it is in accord with a strengthened monsoon in the southern catchment.

(Table 1) Two-way travel times and depths of picked internal reflection horizons (IRHs) of the NGRIP ice core

During the summer of 2003, a ground-penetrating radar survey around the North Greenland Icecore Project (NorthGRIP) deep ice-core drilling site (75°06' N, 42°20' W; 2957 m a.s.l.) was carried out using a shielded 250 MHz radar system. The drill site is located on an ice divide, roughly 300 km north-northwest of the summit of the Greenland ice sheet. More than 430 km of profiles were measured, covering a 10 km by 10 km area, with a grid centered on the drilling location, and eight profiles extending beyond this grid. Seven internal horizons within the upper 120 m of the ice sheet were continuously tracked, containing the last 400 years of accumulation history. Based on the age-depth and density-depth distribution of the deep core, the internal layers have been dated and the regional and temporal distribution of accumulation rate in the vicinity of NorthGRIP has been derived. The distribution of accumulation shows a relatively smoothly increasing trend from east to west from 145 kg/m**2/a to 200 kg/m**2/a over a distance of 50 km across the ice divide. The general trend is overlain by small-scale variations on the order of 2.5 kg/m**2/a/km, i.e. around 1.5% of the accumulation mean. The temporal variations of the seven periods defined by the seven tracked isochrones are on the order of +-4% of the mean of the last 400 years, i.e. at NorthGRIP ±7 kg/m**2/a. If the regional accumulation pattern has been stable for the last several thousand years during the Holocene, and ice flow has been comparable to today, advective effects along the particle trajectory upstream of NorthGRIP do not have a significant effect on the interpretation of climatically induced changes in accumulation rates derived from the deep ice core over the last 10 kyr.

Benthic stable-isotope records of ODP Site 208-1262, South Atlantic

The Late Paleocene and Early Eocene were characterised by warm greenhouse climates, punctuated by a series of rapid warming and ocean acidification events known as "hyperthermals", thought to have been paced or triggered by orbital cycles. While these hyperthermals, such as the Paleocene Eocene Thermal Maximum (PETM), have been studied in great detail, the background low-amplitude cycles seen in carbon and oxygen-isotope records throughout the Paleocene-Eocene have hitherto not been resolved. Here we present a 7.7 million year (myr) long, high-resolution, orbitally-tuned, benthic foraminiferal stable-isotope record spanning the late Paleocene and early Eocene interval (~52.5 - 60.5 Ma) from Ocean Drilling Program (ODP) Site 1262, South Atlantic. This high resolution (~2-4 kyr) record allows the changing character and phasing of orbitally-modulated cycles to be studied in unprecedented detail as it reflects the long-term trend in carbon cycle and climate over this interval. The main pacemaker in the benthic oxygen-isotope (d18O) and carbon-isotope (d13C) records from ODP Site 1262, are the long (405 kyr) and short (100 kyr) eccentricity cycles, and precession (21 kyr). Obliquity (41 kyr) is almost absent throughout the section except for a few brief intervals where it has a relatively weak influence. During the course of the Early Paleogene record, and particularly in the latest Paleocene, eccentricity-paced negative carbon-isotope excursions (d13C, CIEs) and coeval negative oxygen-isotope (d18O) excursions correspond to low carbonate (CaCO3) and coarse fraction (%CF) values due to increased carbonate dissolution, suggesting shoaling of the lysocline and accompanied changes in the global exogenic carbon cycle. These negative CIEs and d18O events coincide with maxima in eccentricity, with changes in d18O leading changes in d13C by ~6 (±5) kyr in the 405-kyr band and by ~3 (±1) kyr in the higher frequency 100-kyr band on average. However, these phase lags are not constant, with the lag in the 405-kyr band extending from ~4 (±5) kyr to ~21 (±2) kyr from the late Paleocene to the early Eocene, suggesting a progressively weaker coupling of climate and the carbon-cycle with time. The higher amplitude 405-kyr cycles in the latest Paleocene are associated with changes in bottom water temperature of 2-4ºC, while the most prominent 100 kyr-paced cycles can be accompanied by changes of up to 1.5ºC. Comparison of the 1262 record with a lower resolution, but orbitally-tuned benthic record for Site 1209 in the Pacific allows for verification of key features of the benthic isotope records which are global in scale including a key warming step at 57.7 Ma.

Depths and ages of seismic sequence boundaries in ODP Leg 166 holes

High-resolution, multichannel seismic data collected across the Great Bahama Bank margin and the adjacent Straits of Florida indicate that the deposition of Neogene-Quaternary strata in this transect are controlled by two sedimentation mechanisms: (1) west-dipping layers of the platform margin, which are a product of sea-level-controlled, platform-derived downslope sedimentation; and (2) east- or north-dipping drift deposits in the basinal areas, which are deposited by ocean currents. These two sediment systems are active simultaneously and interfinger at the toe-of-slope. The prograding system consists of sigmoidal clinoforms that advanced the margin some 25 km into the Straits of Florida. The foresets of the clinoforms are approximately 600 m high with variable slope angles that steepen significantly in the Pleistocene section. The seismic facies of the prograding clinoforms on the slope is characterized by dominant, partly chaotic, cut-and-fill geometries caused by submarine canyons that are oriented downslope. In the basin axis, seismic geometries and facies document deposition from and by currents. Most impressive is an 800-m-thick drift deposit at the confluence of the Santaren Channel and the Straits of Florida. This "Santaren Drift" is slightly asymmetric, thinning to the north. The drift displays a highly coherent seismic facies characterized by a continuous succession of reflections, indicating very regular sedimentation. Leg 166 of the Ocean Drilling Program (ODP) drilled a transect of five deep holes between 2 and 30 km from the modern platform margin and retrieved the sediments from both the slope and basin systems. The Neogene slope sediments consist of peri-platform oozes intercalated with turbidites, whereas the basinal drift deposits consist of more homogeneous, fine-grained carbonates that were deposited without major hiatuses by the Florida Current starting at approximately 12.4 Ma. Sea-level fluctuations, which controlled the carbonate production on Great Bahama Bank by repeated exposure of the platform top, controlled lithologic alternations and hiatuses in sedimentation across the transect. Both sedimentary systems are contained in 17 seismic sequences that were identified in the Neogene-Quaternary section. Seismic sequence boundaries were identified based on geometric unconformities beneath the Great Bahama Bank. All the sequence boundaries could be traced across the entire transect into the Straits of Florida. Biostratigraphic age determinations of seismic reflections indicate that the seismic reflections of sequence boundaries have chronostratigraphic significance across both depositional environments.