Age determination and foraminiferal faunas in sediment core MD99-2275

A high-resolution study of palaeoceanographic changes off North Iceland during the time period 8600-5200 cal year BP is based on benthic and planktonic foraminiferal assemblages. The core material (MD99-2275) was obtained from about 440 m water depth on the eastern part of the North Icelandic shelf. Changes in the faunal composition are interpreted to be mainly caused by variations in the strength of the relatively warm, high-salinity Irminger Current and the cold East Icelandic Current, which have been shown to be linked to the climatic changes in the North Atlantic region. Environmental proxies at that site are particularly sensitive to palaeoceanographic changes due to its position close to the marine Polar Front. Benthic assemblages show that relatively cold conditions prevailed at the base of the record. An increase in the influence of Atlantic water masses at the sea floor is seen at around 8400 cal year BP, whereas the surface waters were relatively warm already at 8600 cal year BP. The warming was interrupted by a cold event at around 8100-8000 cal year BP, registered both in the bottom and surface waters and correlated with the so-called 8.2 kyr cooling event. Both the benthic and the planktonic faunal compositions indicate that the Irminger Current had maximum influence in the area between 8000 and about 7300 cal year BP, followed by a gradually decreasing influence through the remaining part of the studied time interval. It is suggested that the contribution of Atlantic water masses from the east and north-east to the Arctic Surface waters off North Iceland increased after around 7000 cal year BP, and that this was further intensified after 6200 cal year BP. At present, the Arctic Surface Water north of Iceland consists of Polar waters, intermittently with direct influence from the East Greenland Current, mixed with Atlantic waters derived from the eastern part of the Nordic Seas. A comparison of the mean values of selected environmental proxies in the interval 8600-5200 cal year BP with the upper part of the same core shows that the water masses north of Iceland were considerably warmer during the Holocene thermal maximum than during the last 2000 cal year. In general, results from core MD99-2275 are in accordance with other marine records from the North Icelandic shelf and the northern North Atlantic region, although a detailed comparison on a centennial time scale is hampered by problems with spatial as well as temporal changes in the marine reservoir ages in the region.

Multi-proxy analyses of sediment core GeoB13801-2

Over the Uruguayan shelf and uppermost slope the coalescence of northward flowing Subantarctic Shelf Water and southward flowing Subtropical Shelf Water forms a distinct thermohaline front termed the Subtropical Shelf Front (STSF). Running in a SW direction diagonally across the shelf from the coastal waters at 32°S towards the shelf break at ca. 36°S, the STSF represents the shelf-ward extension of the Brazil-Malvinas Confluence zone. This study reconstructs latitudinal STSF shifts during the Holocene based on benthic foraminifera d18O and d13C, total organic carbon, carbonate contents, Ti/Ca, and grain-size distribution from a high-accumulation sedimentary record located at an uppermost continental-slope terrace. Our data provide direct evidence for: (1) a southern STSF position (to the South of the core site) at the beginning of the early Holocene (>9.4 cal ka BP) linked to a more southerly position of the Southern Westerly Winds in combination with restricted shelf circulation intensity due to lower sea level; (2) a gradual STSF northward migration (bypassing the core site towards the North) primarily forced by the northward migration of the Southern Westerly Winds from 9.4 cal ka BP onwards; (3) a relatively stable position of the front in the interval between 7.2 and 4.0 cal ka BP; (4) millennial-scale latitudinal oscillations close to 36°S of the STSF after 4.0 cal ka BP probably linked to the intensification in El Niño Southern Oscillation; and (5) a southward migration of the STSF during the last 200 years possibly linked to anthropogenic influences on the atmosphere.

Sedimentology on two core from the Bransfield Strait area

The raw material for these investigations are samples from marine (sub)surface sediments around the northern part of the Antarctic Peninsula. They had been sampled in the years 1981 to 1986 during several expeditions of the research vessels Meteor, Polarstern and Walther Herwig. 83 box core, gravity core and dredge samples from the area of the Bransfield Strait, the Powell Basin and the northern Weddell Sea have been examined for their grain-size distribution, their mineralogical and petrographical composition. Silt prevails and its clay proportions exceed 25% wt. in water depths greater than 2000 m. The granulometrical results reveal some typical sedimentation processes within the area of investigation. While turbiditic processes together with sediment input from melting icebergs control the sedimentation in the Weddell Sea, the South Orkney Island Plateau and the Powell Basin, the fine grained material from Bransfield Strait mainly relies on marine currents in the shelf area. In addition, the direct sediment input of coarse shelf sediments from the Bransfield Strait into the Powell Basin through submarine canyons could be proven. Variations in the grain-size composition with sediment depth are smalI. The mineral composition of the clay and fine silt fractions is quite uniform in all samples. There are (in decreasing order): illite, montmorillonite, chlorite, smectite, mixed-Iayers, as well as detrital quartz and feldspars. A petrographically based sediment stratigraphy can be established in using the considerable changes in the chlorite- and Ca-plagioclase portions in samples from Core 224. For this sedimentation area a mean sedimentation rate of 7 cm/1000 a is assumed. Remarkable changes in the portions of amorphous silica components - diatom skeletons and volcanic glass shards - appear all over the area of investigation. They contribute between 4-83 % to the clay and fine silt fraction. Several provinces according to the heavy mineral assemblages in the fine sand fraction can be distinguished: (i) a province remarkably influenced by minerals of volcanic origin south and north of the South Shetland Islands; (ii) a small strip with sediment dominated by plutonic material along the western coast of the Antarctic Peninsula and (iii) a sediment controlled by metamorphic minerals and rock fragments in the area of the Weddell Sea and Elephant Island. While taking the whole grain-size spectrum into account a more comprehensive interpretation can be given: the accessoric but distinct appearance of tourmaline, rutile and zircon in the heavy mineral assembly along the northwestern coast of the Antarctic Peninsula is in agreement with the occurrence of acid volcanic rock pieces in the coarse fraction of the ice load detritus in this region. In the vicinity of the South Shetland Islands chlorite appears in remarkable portions in the clay fraction in combination with leucoxene, sphene and olivine, and pumice as well as pyroclastic rocks in the medium and coarse grain fractions, respectively. Amphiboles and amphibole-schists are dominant on the South Orkney Island Plateau. In the sediments of the northwestern Weddell Sea the heavy mineral phases of red spinel, garnet, kyanite and sillimanite in connection with medium to highgrade metamorphic rocks especially granulitic gneisses, are more abundant. A good conformity between the ice rafted rock sampIes and the rocks in the island outcrops could be proven, especially in the vicinity of offshore islands nearby. On the continent enrichments of rock societies and groups appear in spacious outlines: acid effusive rocks in the west of the ice divide on the Antarctic Peninsula, clastic sedimentites at the tip of the Antarctic Peninsula and granoblastic gneisses in central and eastern Antarctica. Coarse grain detritus with more than 1 cm of diameter must have been rafted by icebergs. These rock fragments are classified as rock types, groups and societies. The spacial distribution of their statistically determined weight relations evidently shows the paths of the iceberg drift and in nexus with already known iceberg routes also point to the possible areas of provenance, provided that the density of sample locations and the number of rock pieces are sufficient.

Sedimentary climate indicators of sediment core MD08-3167 and GeoB1711-4, southwestern Africa

The past climate evolution of southwestern Africa is poorly understood and interpretations of past hydrological changes are sometimes contradictory. Here we present a record of leaf-wax dD and View the MathML source taken from a marine sediment core at 23°S off the coast of Namibia to reconstruct the hydrology and C3 versus C4 vegetation of southwestern Africa over the last 140 000 years (140 ka). We find lower leaf-wax dD and higher View the MathML source (more C4 grasses), which we interpret to indicate wetter Southern Hemisphere (SH) summer conditions and increased seasonality, during SH insolation maxima relative to minima and during the last glacial period relative to the Holocene and the last interglacial period. Nonetheless, the dominance of C4 grasses throughout the record indicates that the wet season remained brief and that this region has remained semi-arid. Our data suggest that past precipitation increases were derived from the tropics rather than from the winter westerlies. Comparison with a record from the Congo Basin indicates that hydroclimate in southwestern Africa has evolved in antiphase with that of central Africa over the last 140 ka.

Geochemistry and foraminiferal stable isotoe record of sediment core SCS90-36

Changes in the Southeast Asia monsoon winds and surface circulation patterns since the last glaciation are inferred using multiple paleoceanographic indicators including planktic foraminifer faunal abundances, fauna and alkenones sea-surface temperature (SST) estimates, oxygen and carbon isotopes of planktic and benthic foraminifers, and sedimentary fluxes of carbonates and organic carbon obtained from deep-sea core SCS90-36 from the South China Sea (SCS) (17°59.70'N, 111°29.64'E at water depth 2050 m). All these paleoceanographic evidences indicate marked changes in the SCS ocean system over the last glacial toward the Holocene. Planktic foraminiferal faunal SST estimates show stable warm-season SST of 28.6°C, close to the modern value, throughout the glacial-interglacial cycle. In contrast, cold-season SST increases gradually from 23.6°C in the last glacial to a mean value of 26.4°C in the Holocene with a fluctuation of about 3°C during 13-16 ka. SST estimates by UK'37 method reveal less variability and are in average 1-3°C lower than the fauna-derived winter-season SST. These patterns reveal that the seasonality of the SST is not only higher by about 3-4°C in the glacial, but also a function of the winter season SST. Sedimentation rates decrease from the last glacial-deglacial stage to the Holocene due to a reduction in supply of terrigenous components, which led to an increase of carbonate contents. Total organic carbon (TOC) contents of primarily marine sources decrease from the last glacial-deglacial to the Holocene. The last deglaciation is also characterized by high surface productivity as indicated by increased ketones abundances and high mass accumulation rates (MAR) of the TOC and carbonates. The gradient of planktic foraminifer ocygen and carbon isotopes of between surface dwellers and deep dwellers increases significantly toward Termination I and Holocene, and is indiscernibly small in the carbon isotope gradient of between 14 and 24 ka, revealing a deep-mixing condition in surface layers prior to 10 ka. The glacial-interglacial fluctuation of the carbon isotope value of a benthic foraminifer is 0.61%. which is significantly larger than a global mean value. The large carbon isotope fluctuation indicates an amplification of marginal-sea effects which is most likely resulted from an increase in surface productivity in the northern SCS during the last glacial-deglacial stage. The multiple proxies consistently indicate that the last glacial-deglacial stage winter monsoon in the Southeast Asia was probably strengthened in the northern SCS, leading to a development of deep-mixing surface layer conditions and a more efficient nutrient cycling which supports more marine organic carbon production.

Stable oxygen isotope, UK'37, productivity, CaCO3 and XRF records from sediment core MD06-3075

Proxy records of hydrologic variability in the West Pacific Warm Pool (WPWP) have revealed wide-scale changes in past convective activity in response to orbital and sub-orbital climate forcings. However, attributing proxy responses to regional changes in WPWP hydrology versus local variations in precipitation requires independent records linking the terrestrial and marine realms. We present high-resolution stable isotope, UK'37 sea-surface temperature, X-ray fluorescence (XRF) core scanning and coccolithophore-derived paleoproductivity records covering the past 120 ka from International Marine Global Change (IMAGES) Program Core MD06-3075 (6°29' N, 125°50' E, water depth 1878 m), situated in the Davao Gulf on the southern side of Mindanao. XRF-derived log(Fe/Ca) records provide a robust proxy for runoff-driven sedimentary discharge from Mindanao, whilst past changes in local productivity are associated with variable freshwater runoff and stratification of the surface layer. Significant precessional-scale variability in sedimentary discharge occurred during Marine Isotope Stage (MIS) 5, with peaks in discharge contemporaneous with Northern Hemisphere summer insolation minima. We attribute these changes to the latitudinal migration of the Intertropical Convergence Zone (ITCZ) over the WPWP together with variability in the strength of the Walker circulation acting on precessional timescales. Between 60 and 15 ka sedimentary discharge at Mindanao was muted, displaying little orbital- or millennial-scale variability, likely in response to weakened precessional insolation forcing and lower sea level driving increased subsidence of air masses over the exposed Sunda Shelf. These results highlight the high degree of local variability in the precipitation response to past climate changes in the WPWP.

Stable isotope record and sea surface reconstruction of sediment core MD88-770

Hydrographical changes of the southern Indian Ocean over the last 230 kyr, is reconstructed using a 17-m-long sediment core (MD 88 770; 46°01'S 96°28'E, 3290m). The oxygen and carbon isotopic composition of planktonic (N. pachyderma sinistra and G. bulloides) and benthic (Cibicidoides wuellerstorfi, Epistominella exigua, and Melonis barleeanum) foraminifera have been analysed. Changes in sea surface temperatures (SST) are calculated using diatom and foraminiferal transfer functions. A new core top calibration for the Southern Ocean allows an extension of the method developed in the North Atlantic to estimate paleosalinities (Duplessy et al., 1991). The age scale is built using accelerator mass spectrometry (AMS) 14C dating of N. pachyderma s. for the last 35 kyr, and an astronomical age scale beyond. Changes in surface temperature and salinity clearly lead (by 3 to 7 kyr) deep water variations. Thus changes in deep water circulation are not the cause of the early response of the surface Southern Ocean to climatic changes. We suggest that the early warming and cooling of the Southern Ocean result from at least two processes acting in different orbital bands and latitudes: (1) seasonality modulated by obliquity affects the high-latitude ocean surface albedo (sea ice coverage) and heat transfer to and from the atmosphere; (2) low-latitude insolation modulated by precession influences directly the atmosphere dynamic and related precipitation/ evaporation changes, which may significantly change heat transfer to the high southern latitudes, through their control on latitudinal distribution of the major frontal zones and on the conditions of intermediate and deep water formation.

Age determination and geochemical composition of sediment core DA04-31P

There is limited knowledge pertaining to the history of the Greenland Ice Sheet (GIS) during the last glacial-interglacial transition as it retreated from the continental margins to an inland position. Here we use multiproxy data, including ice-rafted debris (IRD); planktonic isotopes; alkenone temperatures; and tephra geochemistry from the northern Labrador Sea, off southwest Greenland, to investigate the deglacial response of the GIS and evaluate its implications for the North Atlantic deglacial development. The results imply that the southern GIS retreated in three successive stages: (1) early deglaciation of the East Greenland margins, by tephra-rich IRD that embrace Heinrich Event 1; (2) progressive retreat during Allerød culminating in major meltwater releases (d18O depletion of 1.2 per mil) at the Allerød-Younger Dryas transition (12.8-13.0 kyr B.P.); and (3) a final stage of glacial recession during the early Holocene (~9-11 kyr B.P.). Rather than indicating local temperatures of ambient surface water, the alkenones likely were transported to the core site by the Irminger Current. We attribute the timing of GIS retreat to the incursion of warm intermediate waters along the base of grounded glaciers and below floating ice shelves on the continental margin. The results lend support to the view that GIS meltwater presented a forcing factor for the Younger Dryas cooling.

Age determination and model of sediment core RAPiD-15-4P

Greenland ice core records indicate that the last deglaciation (~7-21 ka) was punctuated by numerous abrupt climate reversals involving temperature changes of up to 5°C-10°C within decades. However, the cause behind many of these events is uncertain. A likely candidate may have been the input of deglacial meltwater, from the Laurentide ice sheet (LIS), to the high-latitude North Atlantic, which disrupted ocean circulation and triggered cooling. Yet the direct evidence of meltwater input for many of these events has so far remained undetected. In this study, we use the geochemistry (paired Mg/Ca-d18O) of planktonic foraminifera from a sediment core south of Iceland to reconstruct the input of freshwater to the northern North Atlantic during abrupt deglacial climate change. Our record can be placed on the same timescale as ice cores and therefore provides a direct comparison between the timing of freshwater input and climate variability. Meltwater events coincide with the onset of numerous cold intervals, including the Older Dryas (14.0 ka), two events during the Allerød (at ~13.1 and 13.6 ka), the Younger Dryas (12.9 ka), and the 8.2 ka event, supporting a causal link between these abrupt climate changes and meltwater input. During the Bølling-Allerød warm interval, we find that periods of warming are associated with an increased meltwater flux to the northern North Atlantic, which in turn induces abrupt cooling, a cessation in meltwater input, and eventual climate recovery. This implies that feedback between climate and meltwater input produced a highly variable climate. A comparison to published data sets suggests that this feedback likely included fluctuations in the southern margin of the LIS causing rerouting of LIS meltwater between southern and eastern drainage outlets, as proposed by Clark et al. (2001, doi:10.1126/science.1062517).

Description and geochemical composition of tephra layers from ODP Hole 186-1150A and Site 186-1151

The sediments collected at Sites 1150 and 1151 during Leg 186 included many tephra layers and volcaniclastic detritus. In order to identify these tephras, the major oxide compositions of individual glass shards were determined by electron probe microanalyzer. The uppermost four tephras in sediments from Hole 1150A are correlated with the Towada-Hachinohe tephra (To-H; Tohoku district), Shikotsu Daiichi (1st) tephra (Spfa-1; Hokkaido district), Narugo-Yanagisawa tephra (Nr-Y; Tohoku district), and Aso-4 tephra (Kyushu district), respectively. The uppermost tephra in Hole 1151C is correlated with To-H tephra. To-H, Spfa-1, and Aso-4 tephras are also present in piston core KH94-3, LM-8, collected between Sites 1150 and 1151. Eruptive ages of To-H and Spfa-1 estimated from the oxygen isotopic Stages of core KH94-3, LM-8 are between 14.9-15.3 and 39.5-40.1 ka.

Paleoclimate record combinatio of speleothems and of sediment core GeoB3910-2

A substantial strengthening of the South American monsoon system (SAMS) during Heinrich Stadials (HS) points toward decreased cross-equatorial heat transport as the main driver of monsoonal hydroclimate variability at millennial time-scales. In order to better constrain the exact timing and internal structure of HS1 over tropical South America we assessed two precisely dated speleothem records from central-eastern and northeastern Brazil in combination with two marine records of terrestrial organic and inorganic matter input into the western equatorial Atlantic. During HS1 we recognize at least two events of widespread intensification of the SAMS across the entire region influenced by the South Atlantic Convergence Zone (SACZ) at 16.11-14.69 kyr BP and 18.1-16.66 kyr BP (labeled as HS1a and HS1c, respectively), separated by a dry excursion from 16.66-16.11 kyr BP (HS1b). In view of the spatial structure of precipitation anomalies, the widespread increase of monsoon precipitation over the SACZ domain was termed 'Mega-SACZ'.

Ba, Mn, and Mg record derived from Hoeglundina elegans of sediment core SLA-9

The evolution of productivity, redox conditions, temperature, and ventilation during the deposition of an Aegean sapropel (S1) is independently constrained using bulk sediment composition and high-resolution single specimen benthic foraminiferal trace metal and stable isotope data. The occurrence of benthic foraminifer, Hoeglundina elegans (H. elegans), through a shallow water (260 m) sapropel, permits for the first time a comparison between dissolved and particulate concentrations of Ba and Mn and the construction of a Mg/Ca-based temperature record through sapropel S1. The simultaneous increase in sedimentary Ba and incorporated Ba in foraminiferal test carbonate, (Ba/Ca)H. elegans, points to a close coupling between Ba cycling and export productivity. During sapropel deposition, sedimentary Mn content ((Mn/Al)sed) is reduced, corresponding to enhanced Mn2+ mobilization from sedimentary Mn oxides under suboxic conditions. The consequently elevated dissolved Mn2+ concentrations are reflected in enhanced (Mn/Ca)H. elegans levels. The magnitude and duration of the sapropel interruption and other short-term cooling events are constrained using Mg/Ca thermometry. Based on integrating productivity and ventilation records with the temperature record, we propose a two-mode hysteresis model for sapropel formation.