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.

Mg/Ca-temperature proxy in benthic foraminifera from the Florida Strait

Over the past decade, the ratio of Mg to Ca in foraminiferal tests has emerged as a valuable paleotemperature proxy. However, large uncertainties remain in the relationships between benthic foraminiferal Mg/Ca and temperature. Mg/Ca was measured in benthic foraminifera from 31 high-quality multicore tops collected in the Florida Straits, spanning a temperature range of 5.8° to 18.6°C. New calibrations are presented for Uvigerina peregrina, Planulina ariminensis, Planulina foveolata, and Hoeglundina elegans. The Mg/Ca values and temperature sensitivities vary among species, but all species exhibit a positive correlation that decreases in slope at higher temperatures. The decrease in the sensitivity of Mg/Ca to temperature may potentially be explained by Mg/Ca suppression at high carbonate ion concentrations. It is suggested that a carbonate ion influence on Mg/Ca may be adjusted for by dividing Mg/Ca by Li/Ca. The Mg/Li ratio displays stronger correlations to temperature, with up to 90% of variance explained, than Mg/Ca alone. These new calibrations are tested on several Last Glacial Maximum (LGM) samples from the Florida Straits. LGM temperatures reconstructed from Mg/Ca and Mg/Li are generally more scattered than core top measurements and may be contaminated by high-Mg overgrowths. The potential for Mg/Ca and Mg/Li as temperature proxies warrants further testing.

Numerical modeling of marine radiocarbon reservoir ages during the last deglaciation

A critical problem in radiocarbon dating is the spatial and temporal variability of marine reservoir ages (MRAs). We assessed the MRA evolution during the last deglaciation by numerical modeling, applying a self-consistent iteration scheme in which an existing radiocarbon chronology (derived by Hughen et al., Quat. Sci. Rev., 25, pp. 3216-3227, 2006) was readjusted by transient, 3-D simulations of marine and atmospheric Delta14C. To estimate the uncertainties regarding the ocean ventilation during the last deglaciation, we considered various ocean overturning scenarios which are based on different climatic background states (PD: modern climate, GS: LGM climate conditions). Minimum and maximum MRAs are included in file 'MRAminmax_21-14kaBP.nc'. Three further files include MRAs according to equilibrium simulations of the preindustrial ocean (file 'C14age_preindustrial.nc'; this is an update of our results published in 2005) and of the glacial ocean (files 'C14age_spinupLGM_GS.nc' and 'C14age_spinupLGM_PD.nc').

Reconstruction of sea-surface conditions at middle to high latitudes of the Northern Hemisphere

A new calibration database of census counts of organic-walled dinoflagellate cyst (dinocyst) assemblages has been developed from the analyses of surface sediment samples collected at middle to high latitudes of the Northern Hemisphere after standardisation of taxonomy and laboratory procedures. The database comprises 940 reference data points from the North Atlantic, Arctic and North Pacific oceans and their adjacent seas, including the Mediterranean Sea, as well as epicontinental environments such as the Estuary and Gulf of St. Lawrence, the Bering Sea and the Hudson Bay. The relative abundance of taxa was analysed to describe the distribution of assemblages. The best analogue technique was used for the reconstruction of Last Glacial Maximum (LGM) sea-surface temperature and salinity during summer and winter, in addition to sea-ice cover extent, at sites from the North Atlantic (n=63), Mediterranean Sea (n=1) and eastern North Pacific (n=1). Three of the North Atlantic cores, from the continental margin of eastern Canada, revealed a barren LGM interval, probably because of quasi-permanent sea ice. Six other cores from the Greenland and Norwegian seas were excluded from the compilation because of too sparse assemblages and poor analogue situation. At the remaining sites (n= 54), relatively close modern analogues were found for most LGM samples, which allowed reconstructions. The new LGM results are consistent with previous reconstructions based on dinocyst data, which show much cooler conditions than at present along the continental margins of Canada and Europe, but sharp gradients of increasing temperature offshore. The results also suggest low salinity and larger than present contrasts in seasonal temperatures with colder winters and more extensive sea-ice cover, whereas relatively warm conditions may have prevailed offshore in summer. From these data, we hypothesise low thermal inertia in a shallow and low-density surface water layer.

Mineral dust deposition in interglacial and glacial climate conditions: model results

The global aerosol/climate model ECHAM5-HAM is used in order to investigate the dust cycle for four interglacial and one glacial climate conditions. The 20-year time-slices are the pre-industrial control (CTRL), mid-Holocene (6000 years BP), last glacial inception (115000 years BP), Eemian (126000 years BP) and Last Glacial Maximum (LGM) (21000 years BP) time intervals. The study is focused on the Antarctic region. The model is able to reproduce the magnitude order of dust deposition globally for the pre-industial and LGM climates. Correlation coefficient of the natural logarithm of the observed and modeled values is 0.78 for the CTRL and 0.81 for the LGM. For the pre-industrial simulation the model overestimates observed values in Antarctica by a factor of about 2-3 due to overestimation of the Australian dust source and too high wet deposition in the Antarctica interior. In the LGM, the model underestimates dust deposition in eastern Antarctica by a factor of about 4-5 due to underestimation of the South American dust source. More records are needed to validate dust deposition for the past interglacial time-slices. The modeled results show that dust deposition in Antarctica in the past interglacial time-slices is higher than in the CTRL simulation. The largest increase of dust deposition in Antarctica is simulated for the LGM, showing about 10-fold increase compared to CTRL.

Calcite accumulation in the eastern equatorial Pacific

The eastern equatorial Pacific (EEP) is an important center of biological productivity, generating significant organic carbon and calcite fluxes to the deep ocean. We reconstructed paleocalcite flux for the past 30,000 years in four cores collected beneath the equatorial upwelling and the South Equatorial Current (SEC) by measuring ex230Th-normalized calcite accumulation rates corrected for dissolution with a newly developed proxy for "fraction of calcite preserved". This method produced very similar results at the four sites and revealed that the export flux of calcite was 30-50% lower during the LGM compared to the Holocene. The internal consistency of these results supports our interpretation, which is also in agreement with emerging data indicating lower glacial productivity in the EEP, possibly as a result of lower nutrient supply from the southern ocean via the Equatorial Undercurrent. However, these findings contradict previous interpretations based on mass accumulation rates (MAR) of biogenic material in the sediment of the EEP, which have been taken as reflecting higher glacial productivity due to stronger wind-driven upwelling.

Distribution of pollen and spores in sediments of ODP Hole 175-1078C

The distribution of pollen in marine sediments is used to record vegetation change on the continent. Generally, a good latitudinal correspondence exists between the distribution patterns of pollen in the marine surface sediments and the occurrence of the source plants on the adjacent continent. To investigate land-sea interactions during deglaciation, we compare proxies for continental (pollen assemblages) and marine conditions (alkenone-derived sea surface temperatures) of two high-resolution, radiocarbon-dated sedimentary records from the tropical southeast Atlantic. The southern site is located West of the Cunene River mouth; the northern site is located West of the Angolan Huambe Mountains. It is inferred that the vegetation in Angola developed from Afroalpine and open savannah during the last Glacial maximum (LGM) via Afromontane Podocarpus forest during Heinrich Event 1 (H1), to an early increase of lowland forest after 14.5 ka. The vegetation record indicates dry and cold conditions during the LGM, cool and wet conditions during H1 and a gradual rise in temperature starting well before the Younger Dryas (YD) period. Terrestrial and oceanic climate developments seem largely running parallel, in contrast to the situation ca. 5° further South, where marine and terrestrial developments diverge during the YD. The cool and wet conditions in tropical West Africa, South of the equator, during H1 suggest that low-latitude insolation variation is more important than the slowdown of the thermohaline circulation for the climate in tropical Africa.

Oxygen isotope and Mg/Ca record of planktonic foraminifera of sediment core MD01-2390

Changes in the local freshwater budget over the last 22,000 years have been estimated from a sediment core located in the southern South China Sea (SCS) using a combined approach of Mg/Ca and oxygen isotopes on the planktonic foraminifera Globigerinoides ruber (white) sensu stricto (s.s.). Core MD01-2390 (06°28,12N, 113°24,56E; water depth 1591 m) is located near the glacial paleo-river mouths of the Baram, Rajang and North Sunda/Molengraaff Rivers that drained the exposed Sunda Shelf. The delta18Oseawater record reveals lower average values (-0.96±0.18 per mil) during the Last Glacial Maximum (LGM) when compared with modern values (-0.54±0.18 per mil). Low salinity during the LGM is interpreted to reflect a higher freshwater contribution due to a greater proximity of the core site to the mouths of the Baram, Rajang and North Sunda/Molengraaff Rivers at that time. A general deglacial increasing trend in salinity due to the progressive landward displacement of the coastline during deglacial shelf flooding is punctuated by several short-term shifts towards higher and lower salinity that are likely related to abrupt changes in the intensity of the East Asian summer monsoon. Thus, the deglacial delta18Oseawater changes reflect the combined effects of sea-level-induced environmental changes on the shelf (e.g. phases of retreat and breakdown of the shelf drainage systems) and East Asian monsoon climate change. Lower salinity than at present during the Early Holocene may be attributed to an increase in summer monsoonal precipitation that is corroborated by previous marine and terrestrial studies that report a Preboreal-Early Holocene monsoon optimum in the Asian monsoon region.

Hydrogen isotopic composition of sediment cores from a transect off western Africa

The hydrogen isotopic composition of plant leaf-wax n-alkanes (dDwax) is a novel proxy for estimating dD of past precipitation (dDp). However, vegetation life-form and relative humidity exert secondary effects on dDwax, preventing quantitative estimates of past dDp. Here, we present an approach for removing the effect of vegetation-type and relative humidity from dDwax and thus for directly estimating past dDp. We test this approach on modern day (late Holocene; 0-3 ka) sediments from a transect of 9 marine cores spanning 21°N-23°S off the western coast of Africa. We estimate vegetation type (C3 tree versus C4 grass) using d13C of leaf-wax n-alkanes and correct dDwax for vegetation-type with previously-derived apparent fractionation factors for each vegetation type. Late Holocene vegetation-corrected dDwax (dDvc) displays a good fit with modern-day dDp, suggesting that the effects of vegetation type and relative humidity have both been removed and thus that dDvc is a good estimate of dDp. We find that the magnitude of the effect of C3 tree - C4 grass changes on dDwax is small compared to dDp changes. We go on to estimate dDvc for the mid-Holocene (6-8 ka), the Last Glacial Maximum (LGM; 19-23 ka) and Heinrich Stadial 1 (HS1; 16-18.5 ka). In terms of past hydrological changes, our leaf-wax based estimates of dDp mostly reflect changes in wet season intensity, which is complementary to estimates of wet season length based on leaf-wax d13C.

Past circulation along the western Iberian margin

Fifteen Iberian margin sediment cores, distributed between 43°12'N and 35°53'N, have been used to reconstruct spatial and temporal (sub)surface circulation along the Iberian margin since the Last Glacial period. Time-slice maps of planktonic foraminiferal derived summer sea surface temperature (SST) and export productivity (Pexp) were established for specific time intervals within the last 35 ky: the Holocene (Recent and last 8 ky), Younger Dryas (YD), Heinrich Stadials (HS) 1, 2a, 2b, 3, and the Last Glacial Maximum (LGM). The SST during the Holocene shows the same latitudinal gradient along the western Iberian margin as present-day with cold but productive areas that reflect the influence of coastal upwelling centers. The LGM appears as a slightly less warm, but more productive period relative to the Holocene and present-day conditions, suggesting that sea-level minima forced a westward displacement of the coastal upwelling centers possibly accompanied by a strengthening of northward winds. During the YD, a longitudinal thermal front is depicted at 10°W, with cold polar waters offshore and warmer subtropical waters inshore, suggesting that the subtropical Paleo-Iberian Poleward Current more likely flowed at a more inshore location masking the local SST signal and amplitude of variation. A substantial cooling and drop in productivity is observed during all HS, in particular HS1 and HS3, reflecting the penetration of icebergs-derived meltwater. These most extreme southward extensions of very cold waters define a strong SST gradient that marks a possible Paleo-Azores Front. Higher production south of this front was likely fed by frontal nutrient advection.

Down-core sedimentary Sr-Nd isotope records from two Cape Basin sediment cores in the South Atlantic: GeoB3603-2 (Meteor cruise M34/1) and MD02-2594 (Marion Dufresne cruise MD128)

We report down-core sedimentary Nd isotope (epsilon Nd) records from two South Atlantic sediment cores, MD02-2594 and GeoB3603-2, located on the western South African continental margin. The core sites are positioned downstream of the present-day flow path of North Atlantic Deep Water (NADW) and close to the Southern Ocean, which makes them suitable for reconstructing past variability in NADW circulation over the last glacial cycle. The Fe-Mn leachates epsilon Nd records show a coherent decreasing trend from glacial radiogenic values towards less radiogenic values during the Holocene. This trend is confirmed by epsilon Nd in fish debris and mixed planktonic foraminifera, albeit with an offset during the Holocene to lower values relative to the leachates, matching the present-day composition of NADW in the Cape Basin. We interpret the epsilon Nd changes as reflecting the glacial shoaling of Southern Ocean waters to shallower depths combined with the admixing of southward flowing Northern Component Water (NCW). A compilation of Atlantic epsilon Nd records reveals increasing radiogenic isotope signatures towards the south and with increasing depth. This signal is most prominent during the Last Glacial Maximum (LGM) and of similar amplitude across the Atlantic basin, suggesting continuous deep water production in the North Atlantic and export to the South Atlantic and the Southern Ocean. The amplitude of the epsilon Nd change from the LGM to Holocene is largest in the southernmost cores, implying a greater sensitivity to the deglacial strengthening of NADW at these sites. This signal impacted most prominently the South Atlantic deep and bottom water layers that were particularly deprived of NCW during the LGM. The epsilon Nd variations correlate with changes in 231Pa/230Th ratios and benthic d13C across the deglacial transition. Together with the contrasting 231Pa/230Th: epsilon Nd pattern of the North and South Atlantic, this indicates a progressive reorganization of the AMOC to full strength during the Holocene.

Controls of sediment dynamics on the Galician slope reconstructed from three sediment cores (GeoB11035-1, GeoB130206-1, GeoB13071-1)

Controls of sediment dynamics at the Galician continental slope (NW Iberia) during the past 30 ka were reconstructed from three new gravity cores (GeoB11035-1, 130206-1, 13071-1) based on sedimentological (e.g. sortable silt, IRD), micropalaeontological (e.g. coccoliths), geochemical (AMS 14C, XRF) and geophysical (e.g. magnetic susceptibility) diagnostics. The data are consistent with existing regional knowledge that, during marine isotope stages 3-1, variations in detrital input, marine productivity and sea level were the essential drivers of sediment availability on the slope, whereas deep-water current velocities controlled sediment deposition: (1) the period prior to 30 cal ka BP is characterized by minor but systematic variations in various proxies which can be associated with D-O cycles; (2) between 30 and 18 cal ka BP, high detrital input and steady slope-parallel currents led to constant sedimentation; (3) from the LGM until 10 cal ka BP, the shelf-transgressive sea-level rise increased the detrital particle flux; sedimentation was influenced by significantly enhanced deep-water circulation during the Bølling/Allerød, and subsequent slowing during the Younger Dryas; (4) an abrupt and lasting change to hemipelagic sedimentation at ca. 10 cal ka BP was probably due to Holocene warming and decelerated transgression; (5) after 5 cal ka BP, additional input of detrital material to the slope is plausibly linked to the evolution of fine-grained depocentres on the Galician shelf, this being the first report of this close shelf-slope sedimentary linkage off NW Iberia. Furthermore, there is novel evidence of the nowadays strong outer shelf Iberian Poleward Current becoming established at about 15.5 cal ka BP. The data also demonstrate that small-scale morphologic features and local pathways of sediment export from the neighbouring shelf play an important role for sediment distribution on the NW Iberian slope, including a hitherto unknown sediment conduit off the Ría de Arousa. By implication, the impact of local morphology on along- and down-slope sediment dynamics is more complex than commonly considered, and deserves future attention.

Radiocarbon ages and average physical properties of sediment cores obtained during Icebreaker Oden cruise OSO0910

To date, understanding of ice sheet retreat within Pine Island Bay (PIB) following the Last Glacial Maximum (LGM) was based on seven radiocarbon dates and only fragmentary seafloor geomorphic evidence. During the austral summer 2009-2010, restricted sea ice cover allowed for the collection of 27 sediment cores from the outer PIB trough region. Combining these cores with data from prior cruises, over 133 cores have been used to conduct a detailed sedimentological facies analysis. These results, augmented by 23 new radiocarbon dates, are used to reconstruct the post-LGM deglacial history of PIB. Our results record a clear retreat stratigraphy in PIB composed of, from top to base; terrigenous sandy silt (distal glacimarine), pebbly sandy mud (ice-proximal glacimarine), and till. Initial retreat from the outer-continental shelf began shortly after the LGM and before 16.4 k cal yr BP, as a likely response to rising sea level. Bedforms in outer PIB document episodic retreat in the form of back-stepping grounding zone wedges and are associated with proximal glacimarine sediments. A sub-ice shelf facies is observed in central PIB and spans ~12.3-10.6 k cal yr BP. It is possible that widespread impingement of warm water onto the continental shelf caused an abrupt and widespread change from sub-ice shelf sedimentation to distal glacimarine sedimentation dominated by widespread dispersal of terrigenous silt between 7.8 and 7.0 k cal yr BP. The final phase of retreat ended before ~1.3 k cal yr BP, when the grounding line migrated to a location near the current ice margin.

Age determination and carbonate content of sediment core KH-79-3_L3

Laminated sediments deposited under anoxic bottom waters in the Japan Sea during the last glacial maximum (LGM) contain extremely well preserved calcareous microfossils and eolian carbonates. The radiocarbon age-difference between bulk sediment and monospecific planktonic foraminifera in discrete laminae from a core in the southern Japan Sea implies that ~40% of the total carbonates in the sediments at the LGM are of eolian origin. Extrapolation of this result yields a rate of supply of eolian carbonates of ~2800 tons/d to the entire Japan Sea during the LGM. The climatic significance of this flux potentially lies in its broader geographic extension, particularly in the interaction of the carbonate-bearing dust with shallow, corrosive North Pacific waters and with rain in the atmosphere. By increasing the alkalinity of such waters and by enhancing the biological pump the dust flux could have increased CO2 absorption by both the ocean and rain during the LGM.

Last Glacial Maximum carbon isotopic composition

We present Holocene and last glacial maximum (LGM) oxygen and carbon isotope measurements on Planulina wuellerstorfi in six southeast Pacific cores. Sedimentation rates are low in this part of the ocean, and measurements were made on individual foraminiferal shells in order to identify the Holocene and glacial individuals on the basis of their extreme d18O. The new d13C data were combined with previous P. wuellerstorfi data for interpretation of global thermohaline circulation. Data from the Southern Ocean were examined closely for regional coherency and a few anomalous d13C values suspected of having productivity overprint were removed. The resulting global d13C distributions and gradients indicate that the deep water circulation was similar during the Holocene and LGM. This interpretation brings d13C data to a better agreement with Cd/Ca data and marks a sharp contrast with a widely held view based on d13C measurements that the glacial Southern Ocean was the terminus of the thermohaline circulation. The proposed presence of glacial North Atlantic Deep Water does not necessarily contradict the postulated presence of Glacial North Atlantic Intermediate Water.