Stable isotope ratios from foraminfera in Kara Sea sediments

Ostracods secrete their valve calcite within a few hours or days, therefore, its isotopic composition records ambient environmental conditions of only a short time span. Hydrographic changes between the calcification of individuals lead to a corresponding range (max.-min.) in the isotope values when measuring several (>=5) single valves from a specific sediment sample. Analyses of living (stained) ostracods from the Kara Sea sediment surface revealed high ranges of >2per mil of d18O and d13C at low absolute levels (d18O: <3per mil, d13C: <-3per mil) near the river estuaries of Ob and Yenisei and low ranges of not, vert, similar1per mil at higher absolute levels (d18O: 2-5.4per mil, d13C: -3 per mil to -1.5per mil) on the shelf and in submarine paleo-river channels. Comparison with a hydrographic data base and isotope measurements of bottom water samples shows that the average and the span of the ostracod-based isotope ranges closely mirror the long-term means and variabilities (standard deviation) of bottom water temperature and salinity. The bottom hydrography in the southern part of the Kara Sea shows strong response to the river discharge and its extreme seasonal and interannual variability. Less variable hydrographic conditions are indicative for deeper shelf areas to the north, but also for areas near the river estuaries along submarine paleo-river channels, which act as corridors for southward flowing cold and saline bottom water. Isotope analyses on up to five single ostracod valves per sample in the lower section (8-7 cal. ka BP) of a sediment core north of Yenisei estuary revealed d18O and d13C values which on average are lower by 0.6? in both, d18O and d13C, than in the upper core section (<5 cal. ka BP). The isotope shifts illustrate the decreasing influence of isotopically light river water at the bottom as a result of the southward retreat of the Yenisei river mouth from the coring site due to global sea level rise. However, the ranges (max.-min.) in the single-valve d18O and d13C data of the individual core samples are similar in the upper and in the lower core section, although a higher hydrographic variability is expected prior to 7 cal. ka BP due to river proximity. This lack of variability indicates the southward flow of cold, saline water along a submarine paleo-river channel, formerly existing at the core location. Despite shallowing of the site due to sediment filling of the channel and isostatic uplift of the area, the hydrographic variability at the core location remained low during the Late Holocene, because the shallowing proceeded synchronously with the retreat of the river mouth due to the global sea level rise

Holocene carbon cycle dynamics, links to model files

We are investigating the late Holocene rise in CO2 by performing four experiments with the climate-carbon-cycle model CLIMBER2-LPJ. Apart from the deep sea sediments, important carbon cycle processes considered are carbon uptake or release by the vegetation, carbon uptake by peatlands, and CO 2 release due to shallow water sedimentation of CaCO3. Ice core data of atmospheric CO2 between 8 ka BP and preindustrial climate can only be reproduced if CO2 outgassing due to shallow water sedimentation of CaCO3 is considered. In this case the model displays an increase of nearly 20 ppmv CO2 between 8 ka BP and present day. Model configurations that do not contain this forcing show a slight decrease in atmospheric CO2. We can therefore explain the late Holocene rise in CO2 by invoking natural forcing factors only, and anthropogenic forcing is not required to understand preindustrial CO2 dynamics.

Geochemistry and paleontology of sediment core GeoB3359-3

A geochemical and paleontological reconstruction of paleoproductivity, upwelling intensity and sea surface temperature (SST) off central Chile at 35°S (GeoB3359-3) reveals marked changes from the Last Glacial Maximum (LGM) through the Early Holocene. Surface-water productivity was determined by the interaction between the atmospheric (the Southern Westerlies) and oceanographic (the Antarctic Circumpolar Current, ACC) systems from the LGM through early Termination I (TI). The northward shift of the climate zones during the LGM brought the ACC, as the main macronutrient source, closer to the GeoB3359-3, SST lowered, and surface water productivity and accumulation rates of biogenic components enhanced. With the poleward return of the Southern Westerlies and the ACC, the subtropical high-pressure system became the dominant atmospheric component southward till 35°S during the late TI and Early Holocene and caused surface water productivity to increase through enhanced upwelling.

Age determination of ODP Hole 145-887

Atmospheric carbon dioxide concentrations were significantly lower during glacial periods than during intervening interglacial periods, but the mechanisms responsible for this difference remain uncertain. Many recent explanations call on greater carbon storage in a poorly ventilated deep ocean during glacial periods (Trancois et al., 1997, doi:10.1038/40073; Toggweiler, 1999, doi:10.1029/1999PA900033; Stephens and Keeling, 2000, doi:10.1038/35004556; Marchitto et al., 2007, doi:10.1126/science.1138679; Sigman and Boyle, 2000, doi:10.1038/35038000), but direct evidence regarding the ventilation and respired carbon content of the glacial deep ocean is sparse and often equivocal (Broecker et al., 2004, doi:10.1126/science.1102293). Here we present sedimentary geochemical records from sites spanning the deep subarctic Pacific that -together with previously published results (Keigwin, 1998, doi:10.1029/98PA00874)- show that a poorly ventilated water mass containing a high concentration of respired carbon dioxide occupied the North Pacific abyss during the Last Glacial Maximum. Despite an inferred increase in deep Southern Ocean ventilation during the first step of the deglaciation (18,000-15,000 years ago) (Marchitto et al., 2007, doi:10.1126/science.1138679; Monnin et al., 2001, doi:10.1126/science.291.5501.112), we find no evidence for improved ventilation in the abyssal subarctic Pacific until a rapid transition ~14,600 years ago: this change was accompanied by an acceleration of export production from the surface waters above but only a small increase in atmospheric carbon dioxide concentration (Monnin et al., 2001, doi:10.1126/science.291.5501.112). We speculate that these changes were mechanistically linked to a roughly coeval increase in deep water formation in the North Atlantic (Robinson et al., 2005, doi:10.1126/science.1114832; Skinner nd Shackleton, 2004, doi:10.1029/2003PA000983; McManus et al., 2004, doi:10.1038/nature02494), which flushed respired carbon dioxide from northern abyssal waters, but also increased the supply of nutrients to the upper ocean, leading to greater carbon dioxide sequestration at mid-depths and stalling the rise of atmospheric carbon dioxide concentrations. Our findings are qualitatively consistent with hypotheses invoking a deglacial flushing of respired carbon dioxide from an isolated, deep ocean reservoir periods (Trancois et al., 1997, doi:10.1038/40073; Toggweiler, 1999, doi:10.1029/1999PA900033; Stephens and Keeling, 2000, doi:10.1038/35004556; Marchitto et al., 2007, doi:10.1126/science.1138679; Sigman and Boyle, 2000, doi:10.1038/35038000; Boyle, 1988, doi:10.1038/331055a0), but suggest that the reservoir may have been released in stages, as vigorous deep water ventilation switched between North Atlantic and Southern Ocean source regions.

Reconstruction of the provenance and distribution of terrigenous sediments off NE South America over the last 30ka

We investigate the redistribution of terrigenous materials in the northeastern (NE) South American continental margin during slowdown events of the Atlantic Meridional Overturning Circulation (AMOC). The compilation of stratigraphic data from 108 marine sediment cores collected across the western tropical Atlantic shows an extreme rise in sedimentation rates off the Parnaíba River mouth (about 2°S) during Heinrich Stadial 1 (HS1, 18-15 ka). Sediment core GeoB16206-1, raised offshore the Parnaíba River mouth, documents relatively constant 143Nd/144Nd values (expressed as epsilonNd(0)) throughout the last 30 ka. Whereas the homogeneous epsilonNd(0) data support the input of fluvial sediments by the Parnaíba River from the same source area directly onshore, the increases in Fe/Ca, Al/Si and Rb/Sr during HS1 indicate a marked intensification of fluvial erosion in the Parnaíba River drainage basin. In contrast, the epsilonNd(0) values from sediment core GeoB16224-1 collected off French Guiana (about 7°N) suggest Amazon-sourced materials within the last 30 ka. We attribute the extremely high volume of terrigenous sediments deposited offshore the Parnaíba River mouth during HS1 to (i) an enhanced precipitation in the catchment region and (ii) a reduced North Brazil Current, which are both associated with a weakened AMOC.

Physico-chemical parameters of sediment core PG1972-1 (09-Tik-03) from a Tundra Lake, Northeastern Siberia

Thermokarst lakes in the Siberian Arctic contain sediment archives that can be used for paleoenvironmental inference. Until now, however, there has been no study from the inner Lena River Delta with a focus on diatoms. The objective of this study was to investigate how the diatom community in a thermokarst lake responded to past limnogeological changes and what specific factors drove variations in the diatom assemblage. We analysed fossil diatom species, organic content, grain-size distribution and elemental composition in a sediment core retrieved in 2009 from a shallow thermokarst lake in the Arga Complex, western Lena River Delta. The core contains a 3,000-year record of sediment accumulation. Shifts in the predominantly benthic and epiphytic diatom species composition parallel changes in sediment characteristics. Paleoenvironmental and limnogeological development, inferred from multiple biological and sedimentological variables, are discussed in the context of four diatom zones, and indicate a strong relation between changes in the diatom assemblage and thermokarst processes. We conclude that limnogeological and thermokarst processes such as lake drainage, rather than direct climate forcing, were the main factors that altered the aquatic ecosystem by influencing, for example, habitat availability, hydrochemistry, and water level.

Authigenic uranium, mass accumulation rates, benthic d13C and SST during the last interglacial period of ODP Site 177-1094

Southern Ocean sediments reveal a spike in authigenic uranium 127,000 years ago, within the last interglacial, reflecting decreased oxygenation of deep water by Antarctic Bottom Water (AABW). Unlike ice age reductions in AABW, the interglacial stagnation event appears decoupled from open ocean conditions and may have resulted from coastal freshening due to mass loss from the Antarctic ice sheet. AABW reduction coincided with increased North Atlantic Deep Water (NADW) formation, and the subsequent reinvigoration in AABW coincided with reduced NADW formation. Thus, alternation of deep water formation between the Antarctic and the North Atlantic, believed to characterize ice ages, apparently also occurs in warm climates.

Paleoceanography of sediment cores SO202-27-6 during MIS 3

The modern subarctic Pacific is characterized by a steep salinity-driven surface water stratification, which hampers the supply of saline and nutrient-rich deeper waters into the euphotic zone, limiting productivity. However, the strength of the halocline might have varied in the past. Here, we present diatom oxygen (d18Odiat) and silicon (d30Sidiat) stable isotope data from the open subarctic North-East (NE) Pacific (SO202-27-6; Gulf of Alaska), in combination with other proxy data (Neogloboquadrina pachydermasin d18O, biogenic opal, Ca and Fe intensities, IRD), to evaluate changes in surface water hydrography and productivity during Marine Isotope Stage (MIS) 3, characterized by millennial-scale temperature changes (Dansgaard-Oeschger (D-O) cycles) documented in Greenland ice cores.

Age determination, sedimentological, biogeochemical, and plant macrofossils analyses of outcrop PG2038-1

Thermokarst lakes are a widespread feature of the Arctic tundra, in which highly dynamic processes are closely connected with current and past climate changes. We investigated late Quaternary sediment dynamics, basin and shoreline evolution, and environmental interrelations of Lake El'gene-Kyuele in the NE Siberian Arctic (latitude 71°17'N, longitude 125°34'E). The water-body displays thaw-lake characteristics cutting into both Pleistocene Ice Complex and Holocene alas sediments. Our methods are based on grain size distribution, mineralogical composition, TOC/N ratio, stable carbon isotopes and the analysis of plant macrofossils from a 3.5-m sediment profile at the modern eastern lake shore. Our results show two main sources for sediments in the lake basin: terrigenous diamicton supplied from thermokarst slopes and the lake shore, and lacustrine detritus that has mainly settled in the deep lake basin. The lake and its adjacent thermokarst basin rapidly expanded during the early Holocene. This climatically warmer than today period was characterized by forest or forest tundra vegetation composed of larches, birch trees and shrubs. Woodlands of both the HTM and the Late Pleistocene were affected by fire, which potentially triggered the initiation of thermokarst processes resulting later in lake formation and expansion. The maximum lake depth at the study site and the lowest limnic bioproductivity occurred during the longest time interval of ~7 ka starting in the Holocene Thermal Maximum and lasting throughout the progressively cooler Neoglacial, whereas partial drainage and an extensive shift of the lake shoreline occurred ~0.9 cal. ka BP. Correspondingly, this study discusses different climatic and environmental drivers for the dynamics of a thermokarst basin.

Nd and Pb isotope signatures of the clay-size fraction of sediment core MD99-2227

Nd and Pb isotopes were measured on the fine fraction of one sediment core drilled off southern Greenland. This work aims to reconstruct the evolution of deep circulation patterns in the North Atlantic during the Holocene on the basis of sediment supply variations. For the last 12 kyr, three sources have contributed to the sediment mixture: the North American Shield, the Pan-African and Variscan crusts, and the Mid-Atlantic Ridge. Clay isotope signatures indicate two mixtures of sediment sources. The first mixture (12.2-6.5 ka) is composed of material derived from the North American shield and from a "young" crustal source. From 6.5 ka onward the mixture is characterized by a young crustal component and by a volcanic component characteristic of the Mid-Atlantic Ridge. Since the significant decrease in proximal deglacial supplies, the evolution of the relative contributions of the sediment sources suggests major changes in the relative contributions of the deep water masses carried by the Western Boundary Undercurrent over the past 8.4 kyr. The progressive intensification of the Western Boundary Undercurrent was initially associated mainly with the transport of the Northeast Atlantic Deep Water mass until 6.5 ka and with the Denmark Strait Overflow Water thereafter. The establishment of the modern circulation at 3 ka suggests a reduced influence of the Denmark Strait Overflow Water, synchronous with the full appearance of the Labrador Seawater mass. Our isotopic data set emphasizes several changes in the relative contribution of the two major components of North Atlantic Deep Water throughout the Holocene.

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.

Sedimentaology, pollen profile, and climate reconstruction on a sediment core from Trou Lake, Yukon, Canada

Beringian climate and environmental history are poorly characterized at its easternmost edge. Lake sediments from the northern Yukon Territory have recorded sedimentation, vegetation, summer temperature and precipitation changes since ~16 cal ka BP. Herb-dominated tundra persisted until ~14.7 cal ka BP with mean July air temperatures less than or equal to 5 °C colder and annual precipitation 50 to 120 mm lower than today. Temperatures rapidly increased during the Bølling/Allerød interstadial towards modern conditions, favoring establishment of Betula-Salix shrub tundra. Pollen-inferred temperature reconstructions recorded a pronounced Younger Dryas stadial in east Beringia with a temperature drop of ~1.5 °C (~2.5 to 3.0 °C below modern conditions) and low net precipitation (90 to 170 mm) but show little evidence of an early Holocene thermal maximum in the pollen record. Sustained low net precipitation and increased evaporation during early Holocene warming suggest a moisture-limited spread of vegetation and an obscured summer temperature maximum. Northern Yukon Holocene moisture availability increased in response to a retreating Laurentide Ice Sheet, postglacial sea level rise, and decreasing summer insolation that in turn led to establishment of Alnus-Betula shrub tundra from ~5 cal ka BP until present, and conversion of a continental climate into a coastal-maritime climate near the Beaufort Sea.

Changes in bottom water corrosiveness and carbonate ion concentrations in the sub-Antarctic Atlantic during the last glacial period

The glacial climate system transitioned rapidly between cold (stadial) and warm (interstadial) conditions in the Northern Hemisphere. This variability, referred to as Dansgaard-Oeschger variability, is widely believed to arise from perturbations of the Atlantic Meridional Overturning Circulation. Evidence for such changes during the longer Heinrich stadials has been identified, but direct evidence for overturning circulation changes during Dansgaard-Oeschger events has proven elusive. Here we reconstruct bottom water [CO3]2- variability from B/Ca ratios of benthic foraminifera and indicators of sedimentary dissolution, and use these reconstructions to infer the flow of northern-sourced deep water to the deep central sub-Antarctic Atlantic Ocean. We find that nearly every Dansgaard-Oeschger interstadial is accompanied by a rapid incursion of North Atlantic Deep Water into the deep South Atlantic. Based on these results and transient climate model simulations, we conclude that North Atlantic stadial-interstadial climate variability was associated with significant Atlantic overturning circulation changes that were rapidly transmitted across the Atlantic. However, by demonstrating the persistent role of Atlantic overturning circulation changes in past abrupt climate variability, our reconstructions of carbonate chemistry further indicate that the carbon cycle response to abrupt climate change was not a simple function of North Atlantic overturning.

210Pb and AMS radiocarbon dates of sediment cores JM99-1197 and JM02-59 obtained in Malangen

A high-resolution sedimentary record from the subarctic Malangen fjord in northern Norway, northeastern North Atlantic has been investigated in order to reconstruct variations in influx of Atlantic Water for the last 2000 years. The fjord provides a regional oceanographic climatic signal reflecting changes in the North Atlantic heat flux at this latitude because of its deep sill and the relatively narrow adjoining continental shelf. The reconstructions are based on oxygen and carbon isotopic studies of benthic foraminifera from a high accumulation basin in the Malangen fjord, providing subdecadal time resolution. A comparison between instrumental measurements of bottom water temperatures at the core location and the reconstructed temperatures from benthic foraminiferal d18O for the same time period demonstrates that the stable isotope values reflect the bottom water temperatures very well. The reconstructed temperature record shows an overall decline in temperature of c. 1°C from c. 40 BC to ad 1350. This cooling trend is assumed to be driven by an orbital forced reduction in insolation. Superimposed on the general cooling trend are several periods of warmer or colder temperatures. The long-term fluctuations in the Malangen fjord are concurrent with fluctuations of Atlantic Water in the northern North Atlantic. Although they are not directly comparable, comparisons of atmospheric temperatures and marine records, indicate a close coupling between the climate systems. After ad l800 the record shows an unprecedented warming within the last 2000 years.

Grain size analyses, Ca/Ti and other selected elemental ratios from XRF core scanning of sediment cores from the Apulian Margin

An extensive, high-resolution, sedimentological-geochemical survey was done using geo-acoustics, XRF-core scans, ICP-AES, AMS 14C-dating and grain size analyses of sediments in 11 cores from the Gulf of Taranto, the southern Adriatic Sea, and the central Ionian Sea spanning the last 16 cal. ka BP. Comparable results were obtained for cores from the Gallipoli Shelf (eastern Gulf of Taranto), and the southern Adriatic Sea suggesting that the dominant provenance of Gallipoli Shelf sediments is from the western Adriatic mud belt. The 210Pb and 14C-dated high-accumulation-rate sediments permit a detailed reconstruction of climate variability over the last 16 cal. ka BP. Although, the Glacial-Interglacial transition is generally dry and stable these conditions are interrupted by two phases of increased detrital input during the Bølling-Allerød and the late Younger Dryas. The event during the Younger Dryas period is characterized by increased sediment inputs from southern Italian sources. This suggests that run-off was higher in southern- compared to northern Italy. At approximately ~ 7 cal. ka BP, increased detrital input from the Adriatic mud belt, related to sea level rise and the onset of deep water formation in the Adriatic Sea, is observed and is coincident with the end of sapropel S1 formation in the southern Adriatic Sea. During the mid-to-late Holocene we observed millennial-scale events of increased detrital input, e.g. during the Roman Humid Period, and of decreased detrital input, e.g., Medieval Warm Period. These dry/wet spells are consistent with variability in the North Atlantic Oscillation (NAO). A negative state of the NAO and thus a more advanced penetration of the westerlies into the central Mediterranean, that result in wet conditions in the research area concord with events of high detrital input e.g., during the Roman Humid Period. In contrast, a positive state of the NAO, resulting in dry conditions in the Mediterranean, dominated during events of rapid climate change such as the Medieval Warm Period and the Bronze Age.