Chronostratigraphic framework at sites GeoB14403 and GeoB14414 from the Gela Basin, Sicily Channel

A multi-proxy chronological framework along with sequence-stratigraphic interpretations unveils composite Milankovitch cyclicity in the sedimentary records of the Last GlacialeInterglacial cycle at NE Gela Basin on the Sicilian continental margin. Chronostratigraphic data (including foraminifera-based eco-biostratigraphy and d18O records, tephrochronological markers and 14C AMS radiometric datings) was derived from the shallow-shelf drill sites GeoB14403 (54.6 m recovery) and GeoB14414 (27.5 m), collected with both gravity and drilled MeBo cores in 193 m and 146 m water depth, respectively. The recovered intervals record Marine Isotope Stages and Substages (MIS) from MIS 5 to MIS 1, thus comprising major stratigraphic parts of the progradational deposits that form the last 100-ka depositional sequence. Calibration of shelf sedimentary units with borehole stratigraphies indicates the impact of higher-frequency (20-ka) sea level cycles punctuating this 100-ka cycle. This becomes most evident in the alternation of thick interstadial highstand (HST) wedges and thinner glacial forced-regression (FSST) units mirroring seaward shifts in coastal progradation. Albeit their relatively short-lived depositional phase, these subordinate HST units form the bulk of the 100-ka depositional sequence. Two mechanisms are proposed that likely account for enhanced sediment accumulation ratios (SAR) of up to 200 cm/ka during these intervals: (1) intensified activity of deep and intermediate Levantine Intermediate Water (LIW) associated to the drowning of Mediterranean shelves, and (2) amplified sediment flux along the flooded shelf in response to hyperpycnal plumes that generate through extreme precipitation events during overall arid conditions. Equally, the latter mechanism is thought to be at the origin of undulated features resolved in the acoustic records of MIS 5 Interstadials, which bear a striking resemblance to modern equivalents forming on late-Holocene prodeltas of other Mediterranean shallow-shelf settings.

Radiocarbon, isotope, biogenic, and 230Th measurements for sediment core RC10-196

The subarctic North Pacific Ocean holds a large CO2 reservoir that is currently isolated from the atmosphere by a low-salinity layer. It has recently been hypothesized that the reorganization of these high-CO2 waters may have played a crucial role in the degassing of carbon dioxide to the atmosphere during the last deglaciation. This reorganization would leave some imprint on paleo-productivity records. Here we present 230Th-normalized biogenic fluxes from an intermediate depth sediment core in the Northwest Pacific (RC10-196, 54.7°N, 177.1°E, 1007 m) and place them within the context of a synthesis of previously-published biogenic flux data from 49 deep-sea cores north of 20°N, ranging from 420 to 3968 m water depth. The 230Th-normalized opal, carbonate, and organic carbon fluxes from RC10-196 peak approximately 13,000 calendar years BP during the Bølling/Allerød (B/A) period. Our data synthesis suggests that biogenic fluxes were in general lowest during the last glacial period, increased somewhat in the Northwest Pacific during Heinrich Event 1, and reached a maximum across the entire North Pacific during the B/A period. We evaluate several mechanisms as possible drivers of deglacial change in biogenic fluxes in the North Pacific, including changes in preservation, sediment focusing, sea ice extent, iron inputs, stratification, and circulation shifts initiated in the North Atlantic and North Pacific. Our analysis suggests that while micronutrient sources likely contributed to some of the observed changes, the heterogeneity in timing of glaciogenic retreat and sea level make these mechanisms unlikely causes of region-wide contemporaneous peaks in export production. We argue that paleo-observations are most consistent with ventilation increases in both the North Pacific (during H1) and North Atlantic (during B/A) being the primary drivers of increases in biogenic flux during the deglaciation, as respectively they were likely to bring nutrients to the surface via increased vertical mixing and shoaling of the global thermocline.

Planktonic foraminifera and sea surface temperture reconstruction for Red Sea sediment cores

In order to assess how insolation-driven climate change superimposed on sea level rise and millennial events influenced the Red Sea during the Holocene, we present new paleoceanographic records from two sediment cores to develop a comprehensive reconstruction of Holocene circulation dynamics in the basin. We show that the recovery of the planktonic foraminiferal fauna after the Younger Dryas was completed earlier in the northern than in the central Red Sea, implying significant changes in the hydrological balance of the northern Red Sea region during the deglaciation. In the early part of the Holocene, the environment of the Red Sea closely followed the development of the Indian summer monsoon and was dominated by a circulation mode similar to the current summer circulation, with low productivity throughout the central and northern Red Sea. The climatic signal during the late Holocene is dominated by a faunal transient event centered around 2.4 ka BP. Its timing corresponds to that of North Atlantic Bond event 2 and to a widespread regionally recorded dry period. This faunal transient is characterized by a more productive foraminiferal fauna and can be explained by an intensification of the winter circulation mode and high evaporation. The modern distribution pattern of planktonic foraminifera, reflecting the prevailing circulation system, was established after 1.7 ka BP.

Reconstructed glaciation limits in Eurasia (digitized from map Fig.1)

Based on field investigations in northern Russia and interpretation of offshore seismic data, we have made a preliminary reconstruction of the maximum ice-sheet extent in the Barents and Kara Sea region during the Early/Middle Weichselian and the Late Weichselian. Our investigations indicate that the Barents and Kara ice sheets attained their maximum Weichselian positions in northern Russia prior to 50 000 yr BP, whereas the northeastern flank of the Scandinavian Ice Sheet advanced to a maximum position shortly after 17000 calendar years ago. During the Late Weichselian (25 000-10000 yr BP), much of the Russian Arctic remained ice-free. According to our reconstruction, the extent of the ice sheets in the Barents and Kara Sea region during the Late Weichselian glacial maximum was less than half that of the maximum model which, up to now, has been widely used as a boundary condition for testing and refining General Circulation Models (GCMs). Preliminary numerical-modelling experiments predict Late Weichselian ice sheets which are larger than the ice extent implied for the Kara Sea region from dated geological evidence, suggesting very low precipitation.

Stable isotope and geochemical record of sediments from the Bering Sea

here is controversy over the role of marine methane hydrates in atmospheric methane concentrations and climate change during the last glacial period. In this study of two sediment cores from the southeast Bering Sea (700 m and 1467 m water depth), we identify multiple episodes during the last glacial period of intense methane flux reaching the seafloor. Within the uncertainty of the radiocarbon age model, the episodes are contemporaneous in the two cores and have similar timing and duration as Dansgaard-Oeschger events. The episodes are marked by horizons of sediment containing 13C-depleted authigenic carbonate minerals; 13C-depleted archaeal and bacterial lipids, which resemble those found in ANME-1 type anaerobic methane oxidizing microbial consortia; and changes in the abundance and species distribution of benthic foraminifera. The similar timing and isotopic composition of the authigenic carbonates in the two cores is consistent with a region-wide increase in the upward flux of methane bearing fluids. This study is the first observation outside Santa Barbara Basin of pervasive, repeated methane flux in glacial sediments. However, contrary to the "Clathrate Gun Hypothesis" (Kennett et al., 2003), these coring sites are too deep for methane hydrate destabilization to be the cause, implying that a much larger part of the ocean's sedimentary methane may participate in climate or carbon cycle feedback at millennial timescales. We speculate that pulses of methane in these opal-rich sediments could be caused by the sudden release of overpressure in pore fluids that builds up gradually with silica diagenesis. The release could be triggered by seismic shaking on the Aleutian subduction zone caused by hydrostatic pressure increase associated with sea level rise at the start of interstadials.

Micro-facies and µXRF element scanning data of Dead Sea sediment cores DSEn and 5017-1

Laminated lake sediments from the Dead Sea basin provide high-resolution records of climatic variability in the eastern Mediterranean region, which is especially sensitive to changing climatic conditions. In this study, we aim on detailed reconstruction of climatic fluctuations and related changes in the frequency of flood and dust deposition events at ca. 3300 and especially at 2800 cal. yr BP from high-resolution sediment records of the Dead Sea basin. A ca. 4-m-thick, mostly varved sediment section from the western margin of the Dead Sea (DSEn - Ein Gedi profile) was analysed and correlated to the new International Continental Scientific Drilling Program (ICDP) Dead Sea Deep Drilling Project core 5017-1 from the deep basin. To detect even single event layers, we applied a multi-proxy approach of high-resolution microscopic thin section analyses, micro-X-ray fluorescence (µ-XRF) element scanning and magnetic susceptibility measurements, supported by grain size data and palynological analyses. Based on radiocarbon and varve dating, two pronounced dry periods were detected at ~3500-3300 and ~3000-2400 cal. yr BP which are differently expressed in the sediment records. In the shallow-water core (DSEn), the older dry period is characterised by a thick sand deposit, whereas the sedimentological change at 2800 cal. yr BP is less pronounced and characterised mainly by an enhanced frequency of coarse detrital layers interpreted as erosion events. In the 5017-1 deep-basin core, both dry periods are depicted by halite deposits. The onset of the younger dry period coincides with the Homeric Grand Solar Minimum at ca. 2800 cal. yr BP. Our results suggest that during this period, the Dead Sea region experienced an overall dry climate, superimposed by an increased occurrence of flash floods caused by a change in synoptic weather patterns.

Nitrogen isotopic and opal record of sediments in the Gulf of California

High-resolution records of the nitrogen isotopic composition of organic matter (d15Norg), opal content, and opal accumulation rates from the central Gulf of California reveal large and abrupt variations during deglaciation and gradual Holocene changes coincident with climatic changes recorded in the North Atlantic. Homogenous sediments with relatively low d15Norg values and low opal content were deposited at the end of the last glacial period, during the Younger-Dryas event, and during the middle to late Holocene. In contrast, laminated sediments deposited in the two deglacial stages are characterized by very high d15Norg values (>14 per mil) and opal accumulation rates (29-41 mg/cm**2/yr). Abrupt shifts in d15Norg were driven by widespread changes in the extent of suboxic subsurface waters supporting denitrification and were amplified in the central gulf record due to variations in upwelling, vertical mixing, and/or the latitudinal position of the Intertropical Convergence Zone.

High-resolution marine record of sediment core MD99-2343 in the Balearic Sea

The IMAGES core MD99-2343, recovered from a sediment drift north of the island of Minorca, in the north-western Mediterranean Sea, holds a high-resolution sequence that is perfectly suited to study the oscillations of the overturning system of the Western Mediterranean Deep Water (WMDW). Detailed analysis of grain-size and bulk geochemical composition reveals the sensitivity of this region to climate changes at both orbital and centennial-millennial temporal scales during the last 50 kyr. The dominant orbital pattern in the K/Al record indicates that sediment supply to the basin was controlled by the insolation evolution at 40°N, which forced changes in the fluvial regime, with more efficient sediment transport during insolation maxima. This orbital control also modulated the long-term pattern of the WMDW intensity as illustrated by the silt/clay ratio. However, deep convection was particularly sensitive to climatic changes at shorter time-scales, i.e. to centennial-millennial glacial and Holocene oscillations that are well documented by all the paleocurrent intensity proxies (Si/Al, Ti/Al and silt/clay ratios). Benthic isotopic records (d13C and d18O) show a Dansgaard-Oeschger (D-O) pattern of variability of WMDW properties, which can be associated with changing intensities of the deep currents system. The most prominent reduction on the WMDW overturning was caused by the post-glacial sea level rise. Three main scenarios of WMDW overturning are revealed: a strong mode during D-O Stadials, a weak mode during D-O Interstadials and an intermediate mode during cooling transitions. In addition, D-O Stadials associated with Heinrich events (HEs) have a very distinct signature as the strong mode of circulation, typical for the other D-O Stadials, was never reached during HE due to the surface freshening induced by the inflowing polar waters. Consequently, the WMDW overturning system oscillated around the intermediate mode of circulation during HE. Though surface conditions were more stable during the Holocene, the WMDW overturning cell still reacted synchronously to short-lived events, as shown by increments in the planktonic d18O record, triggering quick reinforcements of the deep water circulation. Overall, these results highlight the sensitivity of the WMDW to rapid climate change which in the recent past were likely induced by oceanographic and atmospheric reorganizations in the North Atlantic region.

Dust flux records from the Subarctic North Pacific

We present a new record of eolian dust flux to the western Subarctic North Pacific (SNP) covering the past 27000 years based on a core from the Detroit Seamount. Comparing the SNP dust record to the NGRIP ice core record shows significant differences in the amplitude of dust changes to the two regions during the last deglaciation, while the timing of abrupt changes is synchronous. If dust deposition in the SNP faithfully records its mobilization in East Asian source regions, then the difference in the relative amplitude must reflect climate-related changes in atmospheric dust transport to Greenland. Based on the synchronicity in the timing of dust changes in the SNP and Greenland, we tie abrupt deglacial transitions in the 230Th-normalized 4He flux record to corresponding transitions in the well-dated NGRIP dust flux record to provide a new chronostratigraphic technique for marine sediments from the SNP. Results from this technique are complemented by radiocarbon dating, which allows us to independently constrain radiocarbon paleoreservoir ages. We find paleoreservoir ages of 745 ± 140 yr at 11653 yr BP, 680 ± 228 yr at 14630 yr BP and 790 ± 498 yr at 23290 yr BP. Our reconstructed paleoreservoir ages are consistent with modern surface water reservoir ages in the western SNP. Good temporal synchronicity between eolian dust records from the Subantarctic Atlantic and equatorial Pacific and the ice core record from Antarctica supports the reliability of the proposed dust tuning method to be used more widely in other global ocean regions.

Age models and stabile isotope ratios from SO35 cores

Stable isotope measurements on the planktonic foraminifer Globigerinoides ruber (white) have been carried out on a number of selected deep-seas sediment cores from the South Lau and Norlh Fiji Basins. The d18O-curves show good correlation with the inter-ocean oraphic correlation composite d18O-record of the standard reference section (Prell et al. 1986), which, in combination with the chronostratigraphic classifications of Herterich & Sarnthein (1984, modified) and Imbrie et al. 1984), allows a detailed dating of the sedimentary sequences. The deepest layers in core no. 119 (southern Lau Basin) could be assigned to Isotope Stage 24. Measurements made on bulk carbonate in two cores show a much higher glacial-interglacial amplitude, allowing the general identification of the conventional oxygen isotope stages. The d13C-values of the benthic foraminifer Cibicidoides wuellerstorfi show progressively lighter values northwards reflecting an increasing contribution of the isotopically lighter CO2 from the remineralisation of organic matter during the general northward movement of the deep water masses. Cyclicities in the sedimentation rates were observed in core nos. 117 and 119 (both southern Lau Basin) where the interglacials exhibit higher levels than the glacials. Calculated new or export paleoproductivity show that the glacials had higher productivity in the euphotic zone. From the oxygen isotope stratigraphy, the five ash layers in core nos. 117 and 119 could be dated as about 530 ka B.P. in Stage 14, 695 ka B.P. in Stage 18, 775 ka B.P. in Stage 21, 790 ka B.P. and 825 ka B.P. in Stage 22. Carbonate dissolution occurred during stages 5, 8 and 10 to 12.

Age determination and stable isotope record of foraminifera of sediment core SO82_5-2

Surface and deepwater paleoclimate records in Irminger Sea core SO82-5 (59°N, 31°W) and Icelandic Sea core PS2644 (68°N, 22°W) exhibit large fluctuations in thermohaline circulation (THC) from 60 to 18 calendar kyr B.P., with a dominant periodicity of 1460 years from 46 to 22 calendar kyr B.P., matching the Dansgaard-Oeschger (D-O) cycles in the Greenland Ice Sheet Project 2 (GISP2) temperature record [Grootes and Stuiver, 1997, doi:10.1029/97JC00880]. During interstadials, summer sea surface temperatures (SSTsu) in the Irminger Sea averaged to 8°C, and sea surface salinities (SSS) averaged to ~36.5, recording a strong Irminger Current and Atlantic THC. During stadials, SSTsu dropped to 2°-4°C, in phase with SSS drops by ~1-2. They reveal major meltwater injections along with the East Greenland Current, which turned off the North Atlantic deepwater convection and hence the heat advection to the north, in harmony with various ocean circulation and ice models. On the basis of the IRD composition, icebergs came from Iceland, east Greenland, and perhaps Svalbard and other northern ice sheets. However, the southward drifting icebergs were initially jammed in the Denmark Strait, reaching the Irminger Sea only with a lag of 155-195 years. We also conclude that the abrupt stadial terminations, the D-O warming events, were tied to iceberg melt via abundant seasonal sea ice and brine water formation in the meltwater-covered northwestern North Atlantic. In the 1/1460-year frequency band, benthic ?18O brine water spikes led the temperature maxima above Greenland and in the Irminger Sea by as little as 95 years. Thus abundant brine formation, which was induced by seasonal freezing of large parts of the northwestern Atlantic, may have finally entrained a current of warm surface water from the subtropics and thereby triggered the sudden reactivation of the THC. In summary, the internal dynamics of the east Greenland ice sheet may have formed the ultimate pacemaker of D-O cycles.

Physical properties of sediment cores from the Labrador Sea

Sediment core logs from six sediment cores in the Labrador Sea show millennial-scale climate variability during the last glacial by recording all Heinrich events and several major Dansgaard-Oeschger cycles. The same millennial-scale climate change is documented for surface-water d18O records of Neogloboquadrina pachyderma (left coiled); hence the surface-water d18O record can be derived from sediment core logging by means of multiple linear regression, providing a paleoclimate proxy record at very high temporal resolution (70 yrs). For the Labrador Sea, sediment core logs contain important information about deep-water current velocities and also reflect the variable input of IRD from different sources as inferred from grain-size analysis, benthic d18O, the relation of density and p-wave velocity, and magnetic susceptibility. For the last glacial, faster deep-water currents which correspond to highs in sediment physical properties, occurred during iceberg discharge and lasted for a several centuries to a few millennia. Those enhanced currents might have contributed to increased production of intermediate waters during times of reduced production of North Atlantic Deep Water. Hudson Strait might have acted as a major supplier of detrital carbonate only during lowered sea level (greater ice extent). During coldest atmospheric temperatures over Greenland, deep-water currents increased during iceberg discharge in the Labrador Sea, then surface water freshened shortly after, while the abrupt atmospheric temperature rise happened after a larger time lag of >=1 kyr. The correlation implies a strong link and common forcing for atmosphere, sea surface, and deep water during the last glacial at millennial time scales but decoupling at orbital time scales.

Bulk geochemical analysis and planktonic foraminifera of sediment core GeoB7112-5

A multivariable approach utilising bulk sediment, planktonic Foraminifera and siliceous phytoplankton has been used to reconstruct rapid variations in palaeoproductivity in the Peru-Chile Current System off northern Chile for the past 19000 cal. yr. During the early deglaciation (19000-16000 cal. yr BP), our data point to strongest upwelling intensity and highest productivity of the past 19 000 cal. yr. The late deglaciation (16000-13000 cal. yr BP) is characterised by a major change in the oceanographic setting, warmer water masses and weaker upwelling at the study site. Lowest productivity and weakest upwelling intensity are observed from the early to the middle Holocene (13000-4000 cal. yr BP), and the beginning of the late Holocene (<4000 cal. yr BP) is marked by increasing productivity, mainly driven by silicate-producing organisms. Changes in the productivity and upwelling intensity in our record may have resulted from a large-scale compression and/or displacement of the South Pacific subtropical gyre during more productive periods, in line with a northward extension of the Antarctic Circumpolar Current and increased advection of Antarctic water masses with the Peru-Chile Current. The corresponding increase in hemispheric thermal gradient and wind stress induced stronger upwelling. During the periods of lower productivity, this scenario probably reversed.