Markus Kienast

High‐resolution UK 37 temperature reconstructions in the South China Sea over the past 220 kyr

Past sea surface temperatures (SST) in the northern and southern areas of the South China Sea have been reconstructed for the past 220 kyr using the UK37 alkenone index. The SST profiles follow the glacial/interglacial pattern exhibiting differences between Last Glacial Maximum and Holocene that are 1°–3°C larger than those observed at the same latitudes in the Atlantic and Pacific Oceans. In Termination I both planktonic foraminiferal δ18O and SST exhibit well-defined Bolling-Allerod and Younger Dryas events with temperature differences between both periods of 0.8° and 0.4°C in north and south, respectively. SSTs record a constant north-south difference of 1°C in the interglacials and nearly 2.5°C in the glacial stages. These differences define two distinct climatic and water circulation patterns that correspond with glacial/interglacial sea level oscillations which opened and closed water exchange with the tropical Indo-Pacific Ocean through the present Sunda Shelf.

Eastern Pacific cooling and Atlantic overturning circulation during the last deglaciation

Surface ocean conditions in the equatorial Pacific Ocean could hold the clue to whether millennial-scale global climate change during glacial times was initiated through tropical ocean–atmosphere feedbacks or by changes in the Atlantic thermohaline circulation. North Atlantic cold periods during Heinrich events and millennial-scale cold events (stadials) have been linked with climatic changes in the tropical Atlantic Ocean and South America, as well as the Indian and East Asian monsoon systems, but not with tropical Pacific sea surface temperatures. Here we present a high-resolution record of sea surface temperatures in the eastern tropical Pacific derived from alkenone unsaturation measurements. Our data show a temperature drop of ∼1 °C, synchronous (within dating uncertainties) with the shutdown of the Atlantic meridional overturning circulation during Heinrich event 1, and a smaller temperature drop of ∼0.5 °C synchronous with the smaller reduction in the overturning circulation during the Younger Dryas event. Both cold events coincide with maxima in surface ocean productivity as inferred from 230Th-normalized carbon burial fluxes, suggesting increased upwelling at the time. From the concurrence of equatorial Pacific cooling with the two North Atlantic cold periods during deglaciation, we conclude that these millennial-scale climate changes were probably driven by a reorganization of the oceans’ thermohaline circulation, although possibly amplified by tropical ocean–atmosphere interaction as suggested before.

A review of nitrogen isotopic alteration in marine sediments

Key Points: Use of sedimentary nitrogen isotopes is examined; On average, sediment 15N/14N increases approx. 2 per mil during early burial; Isotopic alteration scales with water depth Abstract: Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record. However, bulk sedimentary nitrogen isotope ratios, which can be determined routinely and at minimal cost, may be altered during burial and early sedimentary diagenesis, particularly outside of continental margin settings. The causes and detailed mechanisms of isotopic alteration are still under investigation. Case studies of the Mediterranean and South China Seas underscore the complexities of investigating isotopic alteration. In an effort to evaluate the evidence for alteration of the sedimentary N isotopic signal and try to quantify the net effect, we have compiled and compared data demonstrating alteration from the published literature. A >100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that, at sites located off of the continental margins, an increase in sediment 15N/14N occurs during early burial, likely at the seafloor. The extent of isotopic alteration appears to be a function of water depth. Depth-related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of N isotopic alteration. Moreover, the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global N cycle.

Unchanged nitrogen isotopic composition of organic matter in the South China Sea during the last climatic cycle: Global implications

The δ15N 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‰, 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‰ 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 δ15N 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.

An evaluation of 14C age relationships between co‐occurring foraminifera, alkenones, and total organic carbon in continental margin sediments

[ 1] Radiocarbon age relationships between co- occurring planktic foraminifera, alkenones, and total organic carbon in sediments from the continental margins of southern Chile, northwest Africa, and the South China Sea were compared with published results from the Namibian margin. Age relationships between the sediment components are site- specific and relatively constant over time. Similar to the Namibian slope, where alkenones have been reported to be 1000 - 4500 years older than co- occurring foraminifera, alkenones were significantly ( similar to 1000 years) older than co- occurring foraminifera in the Chilean margin sediments. In contrast, alkenones and foraminifera were of similar age ( within 2 sigma error or better) in the NW African and South China Sea sediments. Total organic matter and alkenone ages were similar off Namibia ( age difference TOC alkenones: 200 - 700 years), Chile ( 100 - 450 years), and NW Africa ( 360 - 770 years), suggesting minor contributions of preaged terrigenous material. In the South China Sea, total organic carbon is significantly ( 2000 - 3000 years) older owing to greater inputs of preaged terrigenous material. Age offsets between alkenones and planktic foraminifera are attributed to lateral advection of organic matter. Physical characteristics of the depositional setting, such as seafloor morphology, shelf width, and sediment composition, may control the age of co- occurring sediment components. In particular, offsets between alkenones and foraminifera appear to be greatest in deposition centers in morphologic depressions. Aging of organic matter is promoted by transport. Age offsets are correlated with organic richness, suggesting that formation of organic aggregates is a key process.

Benthic foraminiferal paleoceanography of the South China Sea over the last 40,000 years

Benthic foraminifera in gravity and piston cores from two sites of the northern and southern slopes of the South China Sea (SCS) were analyzed to evaluate changes in surface productivity and deep-water mass characteristics over the last 40,000 years. Our observations suggest that changes in organic carbon flux, that is food supply, and chemical and=or physical properties of the ambient water mass may be the two primary and intercorrelated factors controlling the distribution patterns of benthic foraminifera. When organic carbon flux increased above 3.5 g C m 2 yr 1 in the southern SCS during the last glacial maximum and in the northern SCS during the first part of the Holocene around 10 ka B.P., a group of detritus feeders including Bulimina aculeata and Uvigerina peregrina dominated the benthic foraminiferal assemblage as shown by relative abundance (%) and accumulation rates. This may reflect episodes of increased surface productivity, possibly induced by increased input of nutrients from nearby river runoff. Suspension feeders such as Cibicidoides wuellerstorfiand a group of ‘opportunistic’ species including Oridorsalis umbonatus, Melonis barleeanum and Chilostomella ovoidea gradually became more abundant than detritus feeders as soon as the organic carbon flux decreased to 2.5‐3.5 g C m 2 yr 1 . Similar glacial to interglacial changes in relative abundance and accumulation rates were observed in both cores for a number of species, including Eggerella bradyi, Globocassidulina subglobosa , Astrononion novozealandicum , Sphaeroidina bulloidesand Cibicidoides robertsonianus. These changes were not correlated to the distribution patterns of organic carbon in both cores and may have been related to yet unspecified changes in chemical and=or physical properties of the ambient water mass, independent of changes in organic carbon flux.

Thorium-230 normalized particle flux and sediment focusing in the Panama Basin region during the last 30,000 years

[1] Application of the 230 Th normalization method to estimate sediment burial fluxes in six cores from the eastern equatorial Pacific (EEP) reveals that bulk sediment and organic carbon fluxes display a coherent regional pattern during the Holocene that is consistent with modern oceanographic conditions, in contrast with estimates of bulk mass accumulation rates (MARs) derived from core chronologies. Two nearby sites (less than 10 km apart), which have different MARs, show nearly identical 230 Th-normalized bulk fluxes. Focusing factors derived from the 230 Th data at the foot of the Carnegie Ridge in the Panama Basin are >2 in the Holocene, implying that lateral sediment addition is significant in this part of the basin. New geochemical data and existing literature provide evidence for a hydrothermal source of sediment in the southern part of the Panama Basin and for downslope transport from the top of the Carnegie Ridge. The compilation of core records suggests that sediment focusing is spatially and temporally variable in the EEP. During oxygen isotope stage 2 (OIS 2, from 13–27 ka BP), focusing appears even higher compared to the Holocene at most sites, similar to earlier findings in the eastern and central equatorial Pacific. The magnitude of the glacial increase in focusing factors, however, is strongly dependent on the accuracy of age models. We offer two possible explanations for the increase in glacial focusing compared to the Holocene. The first one is that the apparent increase in lateral sediment redistribution is partly or even largely an artifact of insufficient age control in the EEP, while the second explanation, which assumes that the observed increase is real, involves enhanced deep sea tidal current flow during periods of low sea level stand.

Synchroneity of meltwater pulse 1a and the Bolling warming:New evidence from the South China Sea

A twofold decrease in long-chain n -alcane ( n -nonacosane) concentrations in a downcore record from the northern South China Sea indicates a rapid drop in the supply of terrigenous organic matter to the open South China Sea during the last deglaciation, paralleled by an equally rapid increase in sea-surface temperatures, corresponding with the Bolling warming at 14.7 ka. The sudden drop in terrigenous organic matter delivery to this marginal basin is interpreted to reflect a short-term response of local rivers to rapid sea-level rise, strongly implying that the Bolling warming and the onset of meltwater pulse (MWP) 1a are synchronous. This phase relation contrasts with the widely cited onset of this MWP 1a ca. 14 ka, and implies that previous studies postulating a weakening of deep-water formation in the North Atlantic due to massive meltwater discharge during MWP 1a need to be reevaluated.

The flooding of Sundaland during the last deglaciation: imprints in hemipelagic sediments from the southern South China Sea

Abstract During the last 30 ka, the South China Sea (SCS) experienced pronounced palaeogeographic changes associated with the postglacial sea level rise, which significantly modified the hydrography of this marginal sea. The most crucial effects in the southern part of the basin were the submergence of Sundaland and the opening of the southern channels connecting the SCS to the tropical Indo-Pacific. Isotopic, sedimentological and organic geochemical parameters determined in two sediment cores from the southern SCS, one in the open sea and the other close to the continental shelf (sites 17961 and 17964, respectively) show that the main hydrographical changes during this period were related to critical thresholds in sea level rise. The main changes occurred at about 15–13.5 ky BP, coincident with Meltwater Pulse (MWP) Ia, when sea surface temperatures (SSTs) at both sites experienced a rapid 1.5°C rise, and the clay content and n-nonacosane concentrations dropped significantly. Both trends reflect a rapid retreat of the coastline and an initial flooding of Sundaland at that time. A second important change, starting with the beginning of MWP Ib at about 11.5 ky BP and culminating at 10 ky BP, involved the establishment of modern hydrographic conditions. This is evident from the rapid convergence of the foraminiferal oxygen isotope records and the establishment of Holocene SST values. These results highlight the need to include the flooding/emergence of Sundaland as an important boundary condition in future modelling studies of Asian palaeomonsoons.

A critical review of marine sedimentaryδ13Corg‐pCO2 estimates: New palaeorecords from the South China Sea and a revisit of other low‐latitude δ13Corg‐pCO2 records

In an attempt to understand better the local biogeochemistry of the South China Sea (SCS) and to unravel the contribution of this marginal low-latitude basin to changes in atmospheric CO2 concentrations, we analyzed the carbon isotopic composition of organic matter (δ13Corg) in four sediment cores from throughout the SCS covering the last 220 kyr. Higher values (around −19.5 to −20.5‰) mark glacial stages, while lower values (around −21 to −22.5‰) are characteristic of interglacials. Following well established procedures, the δ13Corg records are converted to local pCO2 estimates. On the basis of these and other low-latitude δ13Corg−pCO2 estimates from the literature, we present a critical evaluation of the use of δ13C of bulk sedimentary organic matter to hindcast past changes in local CO2(aq). Three crucial pitfalls are identified. (1) Given the present inability to quantify precisely the time-varying amount of terrigenous Corg input to marine sediments, absolute values of pCO2 estimates based on bulk sedimentary Corg are questionable. (2) None of the low-latitude sedimentary δ13Corg−pCO2 records shows the expected correlation between temporal changes in upwelling intensity and CO2 estimates, most likely due to the antagonistic influences of CO2(aq) and phytoplankton growth rate on δ13Corg. (3) A detailed comparison of marine δ13Corg−pCO2 records with the Vostok CO2 record reveals significant differences in phasing, specifically at the end of the last deglaciation and during the oxygen isotope stage 5/4 transition. However, in areas where equilibrium between oceanic and atmospheric CO2 occurs, for example the SCS and the Mediterranean, the timing of changes in δ13Corg should agree with the CO2 record from ice cores if δ13Corg is a reliable proxy for changes in CO2(aq). Taken together, the compilation of records presented here cautions the use of δ13Corg as an unambiguous tracer of dissolved molecular CO2 in the surface ocean and calls for a re-evaluation of the role of the low-latitude ocean on temporal changes in atmospheric CO2.