(Table 1) N-isotope record of planktonc foraminifera of sediment core MD02-2550C2

Constraining variations in marine N2-fixation over glacial-interglacial timescales is crucial for determining the role of the marine nitrogen cycle in modifying ocean productivity and climate, yet paleo-records from N2-fixation regions are sparse. Here we present new nitrogen isotope (d15N) records of bulk sediment and foraminifera test-bound (FB) nitrogen extending back to the last ice age from the oligotrophic Gulf of Mexico (GOM). Previous studies indicate a substantial terrestrial input during the last ice age and early deglacial, for which we attempt to correct the bulk sediment d15N using its observed relationship with the C/N ratio. Both corrected bulk and FB-d15N reveal a substantial glacial-to-Holocene decrease of d15N toward Holocene values of around 2.5 per mil, similar to observations from the Caribbean. This d15N change is most likely due to a glacial-to-Holocene increase in regional N2-fixation. A deglacial peak in the FB-d15N of thermocline dwelling foraminifera Orbulina universa probably reflects a whole ocean increase in the d15N of nitrate during deglaciation. The d15N of the surface dwelling foraminifera Globigerinoides ruber and the corrected bulk d15N show little sign of this deglacial peak, both decreasing from last glacial values much earlier than does the d15N of O. universa; this may indicate that G. ruber and bulk N reflect the euphotic zone signal of an early local increase in N2-fixation. Our results add to the evidence that, during the last ice age, the larger iron input from dust did not lead to enhanced N2-fixation in this region. Rather, the glacial-to-Holocene decrease in d15N is best explained by a response of N2-fixation within the Atlantic to the deglacial increase in global ocean denitrification.

Smear slide analysis of sediment core GeoB5836-2

Laminated sediments spanning the last 20,000 years (though not continuously) in the Shaban Deep, a brine-filled basin in the northern Red Sea, were analyzed microscopically and with backscattered electron imagery in order to determine laminae composition with emphasis on the diatomaceous component. Based on this detailed study, we present schematic models to propose paleoflux scenarios for laminae formation at different time-slices. The investigated core (GeoB 5836-2; 26°12.61'N, 35°21.56'E; water depth 1475 m) shows light and dark alternating laminae that are easily distinguishable in the mid-Holocene and at the end of the deglaciation (13-15 ka) period. Light layers are mainly composed of coccoliths, terrigenous material and diatom fragments, while dark layers consist almost exclusively of diatom frustules (monospecific or mixed assemblages). The regularity in the occurrence of coccolith/diatom couplets points to an annual deposition cycle where contrasting seasons and associated plankton blooms are represented (diatoms-fall/winter deposition, coccoliths-summer signal). We propose that, for the past ~15,000 years, the laminations represent two-season annual varves. Strong dissolution of carbonate, with the concomitant loss of the coccolith-rich layer in sediments older than 15 ka, prevents us from presenting a schematic model of annual deposition. However, the diatomaceous component reveals a marked switch in species composition between Last Glacial Maximum (LGM) sediments (dominated by Chaetoceros resting spores) and sediments somewhat younger (18-19 ka; dominated by Rhizosolenia). We propose that different diatom assemblages reflect changing conditions in stratification in the northern Red Sea: Strong stratification conditions, such as during two meltwater pulses at 14.5 and 11.4 ka, are reflected in the sediment by Rhizosolenia layers, while Chaetoceros-dominated assemblages represent deep convection conditions.

(Table 2) Radiocarbon content and 14C AMS ages of planktonic foraminifera from sediment cores Louis_1900 and Louis_2023

North American freshwater runoff records have been used to support the case that climate flickers were caused by shutdowns of the ocean thermohaline circulation (THC) resulting from reversals of meltwater discharges. Inconsistencies in the documentation of these meltwater switches, however, continue to fuel the debate on the cause/s of the oscillatory nature of the deglacial climate. New oxygen and carbon isotope records from the northern Gulf of Mexico depict in exceptional detail the succession of meltwater floods and pauses through the southern routing during the interval 16 to 8.9 ka (14C years BP; ka, kiloannum). The records underscore the bimodal role played by the Gulf of Mexico as a destination of meltwater discharges from the receding Laurentide Ice Sheet. The evidence indicates that the Gulf of Mexico acted as the principal source of superfloods at 13.4, 12.6, and 11.9 ka that reached the North Atlantic and contributed significantly to density stratification, disruption of ocean ventilation, and cold reversals. Gulf of Mexico lapsed into a "relief valve" position in post-Younger Dryas time, when meltwater discharges were rerouted south at 9.9, 9.7, 9.4, and 9.1 ka, thus temporarily interrupting North Atlantic-bound freshwater discharges from Lake Agassiz. The history of meltwater events in the Gulf of Mexico contradicts the model that meltwater flow via the eastern outlets into the North Atlantic disrupted the ocean THC, causing cooling, while diversions to the Gulf of Mexico via the Mississippi River enhanced THC and warming.

Analyses of benthic and planktonic foraminifera during Marine Isotope Stage 5 of ODP Hole 172-1058C

Subtropical Gyres are an important constituent of the ocean-atmosphere system due to their capacity to store vast amounts of warm and saline waters. Here we decipher the sensitivity of the (sub)surface North Atlantic Subtropical Gyre with respect to orbital and millennial scale climate variability between ~140 and 70 ka, Marine Isotope Stage (MIS) 5. Using (isotope)geochemical proxy data from surface and thermocline dwelling foraminifers from Blake Ridge off the west coast of North America (ODP Site 1058) we show that the oceanographic development at subsurface (thermocline) level is substantially different from the surface ocean. Most notably, surface temperatures and salinities peak during the penultimate deglaciation (Termination II) and early MIS 5e, implying that subtropical surface ocean heat and salt accumulation might have resulted from a sluggish northward heat transport. In contrast, maximum thermocline temperatures are reached during late MIS 5e when surface temperatures are already declining. We argue that the subsurface warming originated from intensified Ekman downwelling in the Subtropical Gyre due to enhanced wind stress. During MIS 5a-d a tight interplay of the subtropical upper ocean hydrography to high latitude millennial-scale cold events can be observed. At Blake Ridge, the most pronounced of these high latitude cold events are related to surface warming and salt accumulation in the (sub)surface. Similar to Termination II, heat accumulated in the Subtropical Gyre probably due to a reduced Atlantic Meridional Overturning Circulation. Additionally, a southward shift and intensification of the subtropical wind belts lead to a decrease of on-site precipitation and enhanced evaporation, coupled to intensified gyre circulation. Subsequently, the northward advection of these warm and saline water likely contributed to the fast resumption of the overturning circulation at the end of these high latitude cold events.

(Table 1) Radiocarbon ages and different calibrated ages for sediment core HLY0503-18TC obtained during the HOTRAX expedition

A short sediment core from a local depression forming an intra basin on the Lomonosov Ridge, was retrieved during the Healy-Oden Trans-Arctic Expedition 2005 (HOTRAX). It contains a record of the Marine Isotope Stages (MIS) 1-3 showing exceptionally high abundances of calcareous microfossils during parts of MIS 3. Based on radiocarbon dating, linear sedimentation rates of 7-9 cm/ka persist during the last deglaciation. The Last Glacial Maximum (LGM) is partly characterized by a hiatus. Planktic foraminiferal abundance variations of Neogloboquadrina pachyderma sinistral and calcareous nannofossils reflect changes in Arctic Ocean summer sea ice coverage and probably inflow of subpolar North Atlantic water. Calibration of the radiocarbon ages, using modeled reservoir corrections from previous studies and the microfossil abundance record of the studied core, results in marine reservoir ages of 1400 years or more, at least during the last deglaciation. Paired benthic-planktic radiocarbon dated foraminiferal samples indicate a slow decrease in age difference between surface and bottom waters from the Lateglacial to the Holocene, suggesting circulation and ventilation changes.

(Table 2) Age determination of sediments from the Nordic Seas

A combined record of three cores spanning the last 18 kyr from the northern North Sea is investigated for content of benthic and planktonic foraminifera and stable oxygen isotopes. The paleoenvironmental development through this time period shows an early deglaciation (18-14.4 ka) and the Younger Dryas (12.7-11.5 ka) characterized by arctic/polar conditions and increased ice rafting in the Norwegian Channel. During the Bølling-Allerød period, warm sea surface temperature (9°C) conditions similar to present conditions are inferred, while bottom waters stayed cold (0-1°C) with normal salinity. The Bølling-Allerød period is interrupted twice at 13.9-13.6 ka (Older Dryas) and at 13.0-12.8 ka (Inter-Allerød Cooling Period) by reductions in sea surface temperatures and increased sea ice cover. The beginning of the Holocene period is marked by increases in surface and bottom water temperature. Superimposed on the broad climatic changes through the Holocene, a series of short-lived oscillations in the ocean circulation are recorded. The amplitude of these Holocene events appears larger in the early Holocene (prior to 8 ka) than compared with the remaining part of the Holocene. This amplification can possibly be attributed to a general increased freshwater budget in the North Atlantic at this time during the final stages of the deglaciation of the Laurentide and Scandinavian ice sheets.

Sea surface temperature estimation of sediment core 86014-12PC51

A recently developed technique for determining past sea surface temperatures (SST), based on an analysis of the unsaturation ratio of long chain C37 methyl alkenones produced by Prymnesiophyceae phytoplankton (U37 k' ), has been applied to an upper Quaternary sediment core from the equatorial Atlantic. U37 k' temperature estimates were compared to those obtained from delta18O of the planktonic foraminifer Globigerinoides sacculifer and of planktonic foraminiferal assemblages for the last glacial cycle. The alkenone method showed 1.8°C cooling at the last glacial maximum, about 1/2 to 1/3 of the decrease shown by the isotopic method (6.3°C) and foraminiferal modern analogue technique estimates for the warm season (3.8°C). Warm season foraminiferal assemblage estimates based on transfer functions are out of phase with the other estimates, showing a 1.4°C drop at the last glacial maximum with an additional 0.9°C drop in the deglaciation. Increased alkenone abundances, total organic carbon percentage and foraminiferal accumulation rates in the last glaciation indicate an increase in productivity of as much as 4 times over present day. These changes are thought to be due to increased upwelling caused by enhanced winds during the glaciation. If U37 k' estimates are correct, as much as 50-70% (up to 4.5°C) of estimated delta18O and modern analogue temperature changes in the last glaciation may have been due to changes in thermocline depth, whereas transfer functions seem more strongly influenced by seasonality changes. This indicates these estimates may be influenced as strongly by other factors as they are by SST, which in the equatorial Atlantic was only reduced slightly in the last glaciation.

Sea ice and wind variability during the Holocene in East Antarctica

A high-resolution continuous record of diatom census counts and diatom specific biomarkers in sediment core NBP0101-JPC24 allows assessment of oceanographic and environmental conditions in eastern Prydz Bay during the deglaciation (11 100-9000 cal yr BP) at decadal timescale. Our study improves previous snapshots investigations based on resin-embedded thin sections and presents a new proxy that compliments the diatom census counts. Our results suggest that the ice sheet retreat over the core site is dated at ~11 100 cal yr BP, setting the onset of local deglaciation and subsequent open marine conditions. The glacial retreat in Prydz Bay is due to global warming initiated at 18 cal ka BP and the regional development of the Prydz Bay cyclonic gyre. Our results further demonstrate that the deglaciation in eastern Prydz Bay can be separated in four phases: the first between 11 100 and 10 900 cal yr BP when the ice shelf was proximal and sea ice was almost perennial; the second and the third phases between 10 900-10 400 cal yr BP and 10 400-9900 cal yr BP, respectively, when the ice shelf retreated and seasonal sea ice cycle consequently developed promoting warmer water to pump into the bay within the gyre, which in turn forced the ice shelf recession and the yearly sea ice cycle establishment; and the fourth between 9900 and 9000 cal yr BP when Holocene condition were set with a recurrent seasonal sea ice cycle and a well established Prydz Bay gyre.

(Table 1) Age determination of sediment core BC5, Golf of Lion

Hitherto unknown abundance peaks of left coiling (l.c.) Neogloboquadrina pachyderma from a Gulf of Lions piston core indicate that abrupt cold spells associated with Atlantic Heinrich events affected the Mediterranean. N. pachyderma (l.c.) is typical of (sub) polar waters in the open ocean. The southern edge of its glacial North Atlantic bioprovince reached south Portugal. Only trace abundances of N. pachyderma (l.c.) are known from Quaternary Mediterranean sediments, suggesting that no significant "invasions" occured via the Strait of Gibraltar. The Gulf of Lions abundance peaks therefore seem to reflect area-specific thriving of a normally rare but indigenous taxon in the western Mediterranean through local favorable habitat development. The general planktonic foraminiferal record suggests that the basic hydrographic regime in the Gulf of Lions, with wintertime deep convective overturn, was relatively stable over the past 60 kyr. Under these conditions, high abundances of N. pachyderma (l.c.) would essentially imply temperature reductions of the order of 5°-8° relative to the present.

Stable oxygen isotope record of benthic foraminifera from the tropical Atlantic

Benthic foraminiferal oxygen isotope ratios from two sediment cores recovered at 426 and 1299 m water depth in the eastern and western tropical Atlantic show that a slowdown of the thermohaline circulation (THC) during Heinrich event H1 and the Younger Dryas was accompanied by rapid and intense warming of intermediate depth waters. Millennial-scale covariations of low paleosalinities in the subpolar North Atlantic with decreased benthic oxygen isotope ratios in the eastern tropical Atlantic throughout the past 10,000 years suggest that THC weakening might be related to middepth warming during the Holocene period as well. Climate model experiments simulating a strong reduction of the THC in the Atlantic Ocean under present-day and glacial conditions reveal that the increase of temperature in the middepth tropical and South Atlantic is a common feature for both climatic states, caused by a reduced ventilation of cold intermediate and deep waters in conjunction with downward mixing of heat from the thermocline. From the similarity of the paleoclimatic records with the model simulations, we infer that the characteristic pattern of temperature change in the Atlantic Ocean related to weakened thermohaline circulation can serve as an indicator of present-day and future THC slowdown.

Sea surface temperature reconstruction for sediment core M35003-4 in the western tropical North Atlantic

Planktonic foraminiferal census counts are used to construct high-resolution sea surface temperature (SST) and subsurface (thermocline) temperature records at a core site in the Tobago Basin, Lesser Antilles. The record is used to document climatic variability at this tropical site in comparison to middle- and high-latitude sites and to test current concepts of cross-equatorial heat transports as a major player in interhemispheric climate variability. Temperatures are estimated using transfer function and modern analog techniques. Glacial - maximum cooling of 2.5°-3°C is indicated; maximum cooling by 4°C is inferred for isotope stage 3. The SST record displays millennial-scale variability with temperature jumps of up to 3°C and closely tracks the structure of ice-core Dansgaard/Oeschger cycles. SST variations in part of the record run opposite to the SST evolution at high northern latitude sites, pointing to thermohaline circulation and marine heat transport as an important factor driving SST in the tropical and high-latitude Atlantic, both on orbital and suborbital timescales.