Sea-surface temperature reconstruction for sediment core GeoB1023-5

Sediment core GeoB 1023-5 from the eastern South Atlantic was investigated at high temporal resolution for variations of sea-surface temperature (SST) during the past 22 kyr, using the alkenone (UK'37) method. SSTs increased by 3.5°C from about 18°C during the Last Ice Age (21±2 cal kyr BP) to about 21.5°C at 14.5 cal kyr BP. This warming trend associated with the deglaciation phase was followed by a cooling event with lowest SSTs near 20°C, persisting for about 1000 years between 13 and 12 cal kyr BP. The SSTs then continued to increase to about 22.5°C at the Holocene climatic optimum at 7 cal kyr BP, and decreased again during the Late Holocene to a core-top value of 19.8°C that is comparable to modern annual mean SST values. When compared with alkenone SST records from the eastern North Atlantic, our SST record indicates continuous warming throughout the deglaciation phase in the Benguela Current, while its northern counterpart, the Canary Current, experienced prominent cooling during 'Heinrich Event 1' (H1). On the other hand, for the time period corresponding to the 'Younger Dryas' (YD) cooling event, the Benguela SST record exhibits a cold-temperature interval that corresponds to that observed in the eastern North Atlantic SST records. This observation suggests that interhemispheric climate response in Atlantic eastern boundary current systems was different with respect to the two abrupt climate events associated with Termination I. For the H1, the eastern South Atlantic SST record strongly supports the hypothesis that an 'anti-phase' thermal behavior in South Atlantic surface waters was forced by the slowdown of the North Atlantic Deep Water formation during cold spells in the North Atlantic. In contrast, the abrupt cooling in the eastern South Atlantic coincident with the YD period was probably induced by more vigorous global atmospheric circulation, enhancing the upwelling intensity in both eastern boundary current systems. This atmospheric control may have overridden any effect caused by changes in thermohaline circulation on the South Atlantic SSTs during the YD, which leads to the assumption that the thermohaline circulation was already much closer to its interglacial mode during the YD than during the H1.

(Table 1) Age determination of sediment core PS51/092-12

Diatom assemblages were employed to study temporal changes of Siberian river runoff on the Laptev Sea shelf. Using a correlation between freshwater diatoms (%) in core-top sediments and summer surface water salinities from the inner Kara Sea, salinity conditions were reconstructed for a site northeast of the Lena River Delta (present water depth 32 m) since 9 calendar years (cal) ka. The reconstruction indicate a strong, near-coastal, and river-influenced environment at the site until about 8.6 cal ka. Corroborated by comparison with other proxy records from further to the east, surface salinities increased from 9 to 14 until about 7.4 cal ka, owing to ongoing global sea level rise and synchronous southward shift of the coastline. Although riverine water became less influential at the site since then, salinities still varied between 12.5 and 15, particularly during the last 3.5 kyr. These more recent salinity fluctuations agree well with reconstructions from just north of the Lena Delta, emphasizing the strong linkage between shelf hydrography and riverine discharge patterns in Arctic Siberia.

Radiocarbon dates, grain size measurements and selected foraminifera analysis from sediment cores AvH248260-2 and AvH248260-1, Ameralik Fjord, SW Greenland

Sedimentological and geochemical (XRF) data together with information from diatom and benthic foraminiferal records of a 3.5 m long gravity core from Ameralik Fjord, southern West Greenland, is used for reconstructing late-Holocene environmental changes in this area. The changes are linked to large-scale North Atlantic ocean and climate variability. AMS 14C-dating of benthic foraminifera indicates that the sediment core records the last 4400 years and covers the termination of the Holocene Thermal Maximum (HTM). The late HTM (4.4 3.2 ka BP) is characterized by high accumulation rates of fine (silty) sediments related to strong meltwater discharge from the Inland Ice. The HTM benthic foraminiferal fauna demonstrates the presence of well-ventilated, saline bottom water originating from inflow of subsurface West Greenland Current water of Atlantic (Irminger Sea) origin. The hydrographic conditions were further characterized by limited sea ice probably related to a mild and relatively windy winter climate. After 3.2 ka BP lower fine-grained sedimentation rates, but a larger input from sea-ice rafted or aeolian coarse material prevailed. This can be related to colder atmospheric conditions with a decreased meltwater discharge and more widespread sea-ice cover in the fjord.

Sea surface temperature reconstruction based on alkenones of sediment core M35003-4

Evidence for abrupt climate changes on millennial and shorter timescales is widespread in marine and terrestrial climate records (Dansgard et al., 1993, doi:10.1038/364218a0; Bond et al., 1993, doi:10.1038/365143a0; Charles et al., 1996, doi:10.1016/0012-821X(96)00083-0, Bard et al., 1997, doi:10.1038/385707a0). Rapid reorganization of ocean circulation is considered to exert some control over these changes (Broecker et al., 1985, doi:10.1038/315021a0), as are shifts in the concentrations of atmospheric greenhouse gases (Broecker, 1994, doi:10.1038/372421a0). The response of the climate system to these two influences is fundamentally different: slowing of thermohaline overturn in the North Atlantic Ocean is expected to decrease northward heat transport by the ocean and to induce warming of the tropical Atlantic (Crowley, 1992, doi:10.1029/92PA01058; Manabe and Stouffer, 1997, doi:10.1029/96PA03932), whereas atmospheric greenhouse forcing should cause roughly synchronous global temperature changes (Manabe et al., 1991, doi:10.1175/1520-0442(1991)004<0785:TROACO>2.0.CO;2). So these two mechanisms of climate change should be distinguishable by the timing of surface-water temperature variations relative to changes in deep-water circulation. Here we present a high-temporal-resolution record of sea surface temperatures from the western tropical North Atlantic Ocean which spans the past 29,000 years, derived from measurements of temperature-sensitive alkenone unsaturation in sedimentary organic matter. We find significant warming is documented for Heinrich event H1 (16,900-15,400 calendar years bp) and the Younger Dryas event (12,900-11,600 cal. yr bp), which were periods of intense cooling in the northern North Atlantic. Temperature changes in the tropical and high-latitude North Atlantic are out of phase, suggesting that the thermohaline circulation was the important trigger for these rapid climate changes.

Bulk sediment element analysis of sediment cores GeoB10065-9 and GeoB10065-7, offshore northwest Sumba Island, Indonesia

The Australian-Indonesian monsoon has a governing influence on the agricultural practices and livelihood in the highly populated islands of Indonesia. However, little is known about the factors that have influenced past monsoon activity in southern Indonesia. Here, we present a ~6000 years high-resolution record of Australian-Indonesian summer monsoon (AISM) rainfall variations based on bulk sediment element analysis in a sediment archive retrieved offshore northwest Sumba Island (Indonesia). The record suggests lower riverine detrital supply and hence weaker AISM rainfall between 6000 yr BP and ~3000 yr BP compared to the Late Holocene. We find a distinct shift in terrigenous sediment supply at around 2800 yr BP indicating a reorganization of the AISM from a drier Mid Holocene to a wetter Late Holocene in southern Indonesia. The abrupt increase in rainfall at around 2800 yr BP coincides with a grand solar minimum. An increase in southern Indonesian rainfall in response to a solar minimum is consistent with climate model simulations that provide a possible explanation of the underlying mechanism responsible for the monsoonal shift. We conclude that variations in solar activity play a significant role in monsoonal rainfall variability at multi-decadal and longer timescales. The combined effect of orbital and solar forcing explains important details in the temporal evolution of AISM rainfall during the last 6000 years. By contrast, we find neither evidence for volcanic forcing of AISM variability nor for a control by long-term variations in the El Niño-Southern Oscillation (ENSO).

Stable carbon isotopes of Planulina wuellerstorfi of sediment core MD01-2444

The observation that Greenland and Antarctic temperatures have followed a specific 'asymmetrical' pattern on millennial time-scales sets rigid constraints on any viable theory of abrupt climate change. The further observation that the very same asymmetry is also reflected in planktonic and benthic d18O measurements from the Northeast Atlantic has extended this constraint to include a specific response in the ocean. Here we present records of deep-water temperature, d18O and d13C variability from the Northeast Atlantic that help to shed light on the links between overturning circulation perturbations, sea-level variability and inter-hemispheric climate change on millennial time-scales. Results indicate that while deep-water temperatures in the Northeast Atlantic have tracked Greenland climate, the d18O signature of local deepwater (d18Odw) has varied in a manner more reminiscent of Antarctic temperature variability. The previously identified correspondence of Antarctic warm events with benthic d18O minima in the Northeast Atlantic is thus found to apply specifically to d18Odw minima, and to extend beyond Marine Isotope Stage 3 to the entirety of the last 50 ka. It is impossible to reconcile completely the Iberian Margin d18Odw record with existing reconstructions of millennial sea-level variability, leading to the conclusion that a significant portion of the d18Odw record must represent local hydrographic change. This is supported by benthic d13C measurements, which suggest the incursion during Greenland stadials of a colder, low-d18O and low-d13C water-mass, of presumed Antarctic origin. These observations confirm a one-to-one coupling of inter-hemispheric climate events with changes in the Atlantic overturning circulation, but fail to rule in or out a unique mechanism by which they were triggered.

Stable oxygen isotope and Mg/Ca ratios of planktonic foraminifera from the Caribbean and North Atlantic

Variations in the strength of the North Atlantic Ocean thermohaline circulation have been linked to rapid climate changes during the last glacial cycle through oscillations in North Atlantic Deep Water formation and northward oceanic heat flux. The strength of the thermohaline circulation depends on the supply of warm, salty water to the North Atlantic, which, after losing heat to the atmosphere, produces the dense water masses that sink to great depths and circulate back south. Here we analyse two Caribbean Sea sediment cores, combining Mg/Ca palaeothermometry with measurements of oxygen isotopes in foraminiferal calcite in order to reconstruct tropical Atlantic surface salinity during the last glacial cycle. We find that Caribbean salinity oscillated between saltier conditions during the cold oxygen isotope stages 2, 4 and 6, and lower salinities during the warm stages 3 and 5, covarying with the strength of North Atlantic Deep Water formation. At the initiation of the Bølling/Allerød warm interval, Caribbean surface salinity decreased abruptly, suggesting that the advection of salty tropical waters into the North Atlantic amplified thermohaline circulation and contributed to high-latitude warming.

Results of Heinrich event 1 simulation from the University of Victoria Earth System-Climate Model (UVic ESCM) and the Community Climate System Model (CCSM3)

We investigated changes in tropical climate and vegetation cover associated with abrupt climate change during Heinrich Event 1 (HE1, ca. 17.5 ka BP) using two different global climate models: the University of Victoria Earth System-Climate Model (UVic ESCM) and the Community Climate System Model version 3 (CCSM3). Tropical South American and African pollen records suggest that the cooling of the North Atlantic Ocean during HE1 influenced the tropics through a southward shift of the rain belt. In this study, we simulated the HE1 by applying a freshwater perturbation to the North Atlantic Ocean. The resulting slowdown of the Atlantic Meridional Overturning Circulation was followed by a temperature seesaw between the Northern and Southern Hemispheres, as well as a southward shift of the tropical rain belt. The shift and the response pattern of the tropical vegetation around the Atlantic Ocean were more pronounced in the CCSM3 than in the UVic ESCM simulation. For tropical South America, opposite changes in tree and grass cover were modeled around 10° S in the CCSM3 but not in the UVic ESCM. In tropical Africa, the grass cover increased and the tree cover decreased around 15° N in the UVic ESCM and around 10° N in the CCSM3. In the CCSM3 model, the tree and grass cover in tropical Southeast Asia responded to the abrupt climate change during the HE1, which could not be found in the UVic ESCM. The biome distributions derived from both models corroborate findings from pollen records in southwestern and equatorial western Africa as well as northeastern Brazil.

Lake sediments from Store Koldewey, Northeast Greenland

A 2.9 m long sedimentary record was studied from a small lake, here referred to as Duck Lake, located at 76°25'N, 18°45'W on Store Koldewey, an elongated island off the coast of Northeast Greenland. The sediments were investigated for their geophysical and biogeochemical characteristics, and for their fossil chironomid assemblages. Organic matter began to accumulate in the lake at 9.1 cal. kyr BP, which provides a minimum age for the deglaciation of the basin. Although the early to mid-Holocene is known as a thermal maximum in East Greenland, organic matter accumulation in the lake remained low during the early Holocene, likely due to late plant immigration and lack of nutrient availability. Organic matter accumulation increased during the middle and late Holocene, when temperatures in East Greenland gradually decreased. Enhanced soil formation probably led to higher nutrient availability and increased production in the lake. Chironomids are abundant throughout the record after 9.1 cal. kyr BP and seem to react sensitively to changes in temperature and nutrient availability. It is concluded that relative temperature reconstructions based on biogeochemical data have to be regarded critically, particularly in the period shortly after deglaciation when nutrient availability was low. Chironomids may be a suitable tool for climatic reconstructions even in those high arctic environments. However, a better understanding of the ecology of chironomids under these extreme conditions is needed.

High-resolution pollen record of ODP Hole 175-1078C

To address the connection between tropical African vegetation development and high-latitude climate change we present a high-resolution pollen record from ODP Site 1078 (off Angola) covering the period 50-10 ka BP. Although several tropical African vegetation and climate reconstructions indicate an impact of Heinrich Stadials (HSs) in Southern Hemisphere Africa, our vegetation record shows no response. Model simulations conducted with an Earth System Model of Intermediate Complexity including a dynamical vegetation component provide one possible explanation. Because both precipitation and evaporation increased during HSs and their effects nearly cancelled each other, there was a negligible change in moisture supply. Consequently, the resulting climatic response to HSs might have been too weak to noticeably affect the vegetation composition in the study area. Our results also show that the response to HSs in southern tropical Africa neither equals nor mirrors the response to abrupt climate change in northern Africa.

Age model, radiocarbon dates and dinoflagellate cyst measurements from sediment core DA06-139G

A marine sediment core from Vaigat in Disko Bugt, West Greenland, has been analysed in terms of lithology, dinoflagellate cysts and foraminifera in order to evaluate the influence of oceanographic variability on West Greenland glacier stability. The data show that during the past 5200 years the Atlantic foraminiferal abundance in the subsurface waters of the West Greenland Current (WGC) episodically increased, indicating periods of increases in the inflow of subsurface warm Atlantic water at 2000 - 1500 cal. yr BP and 1300 cal. yr BP as well as periods of less pronounced increased bottom-water temperatures around 4700 - 4000 cal. yr BP, 3100 - 2800, 2600, 1000 - 800, 500 - 400, and at 200 cal. yr. The sedimentological and dinoflagellate cyst data indicate that these episodes with enhanced advection of Irminger Sea-derived waters are accompanied by increased iceberg rafting, which we link to increased iceberg calving in relation to destabilization of the Jakobshavn Isbrae. The long-term trend in the data documents the end of a late-Holocene Thermal Maximum between 5200 and 4300 cal. yr BP and a final onset of the Neoglaciation at 3500 cal. yr BP. Increased responses of the iceberg rafting after 3500 cal. yr BP, reflects a westward/seaward advance of the glacier margin in relation to onset of Neoglaciation and a development of the glacier into a floating tongue after 2000 cal. yr BP. A comparison of our record with a record from the eastern North Atlantic indicates that a NAO-like anomaly pattern between subsurface waters in West Greenland and atmospheric temperature in the Eastern North Atlantic may have been operating during most of the late Holocene. However, during the past 1000 years the NAO signal may have weakened as some other mode of climate variability overprints the anti-phase climate signal in this region.

Sedimentology and paleontology on core Lz1104 from Lake Hjort, Greenland

Two sediment cores of 70 and 252 cm length were recovered from Hjort Sø, a small lake on Store Koldewey, Northeast Greenland, and studied with a multidisciplinary approach in order to reconstruct the local environmental history and to test the relevance of proxies for paleoenvironmental information. The basal sediments from the longer core are dominated by clastic matter, which was likely deposited during deglaciation of the lake basin. These clastic sediments are overlain by gyttja, which is also present throughout the shorter core. AMS radiocarbon dating was conducted on plant macrofossils of 11 samples from the gyttja in both cores. A reliable chronology was established for both cores, which dated the onset of organic accumulation at 9,500 cal. year BP. The Holocene temperature development, with an early to mid Holocene thermal maximum, is best reflected in the grain-size composition. Nutrient availability was apparently low during the early Holocene and led to low productivity in the lake and its vicinity. From ca. 7,000 cal. year BP, productivity in the lake increased significantly, probably induced by external nutrient input from goose excrements. From this time, micro- and macrofossil remains reflect relatively well the climate history of East Greenland, with a cooling during the middle Holocene, the medieval warming, and the Little Ice Age. The amount of organic matter in the sequence seems to be more affected by lake ice cover or by nutrient supply from the catchment than by temperature changes. The record from Hjort Sø thus reveals the difficulties in interpreting sedimentary records from high arctic regions.