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.

Meteorological measurements from 8 dropsondes released during POLAR 4 flight on 1993-03-04 along a track orthogonal to the pack ice edge north west of Svalbard

Different parameterizations of subgrid-scale fluxes are utilized in a nonhydrostatic and anelastic mesoscale model to study their influence on simulated Arctic cold air outbreaks. A local closure, a profile closure and two nonlocal closure schemes are applied, including an improved scheme, which is based on other nonlocal closures. It accounts for continuous subgrid-scale fluxes at the top of the surface layer and a continuous Prandtl number with respect to stratification. In the limit of neutral stratification the improved scheme gives eddy diffusivities similar to other parameterizations, whereas for strong unstable stratifications they become much larger and thus turbulent transports are more efficient. It is shown by comparison of model results with observations that the application of simple nonlocal closure schemes results in a more realistic simulation of a convective boundary layer than that of a local or a profile closure scheme. Improvements are due to the nonlocal formulation of the eddy diffusivities and to the inclusion of heat transport, which is independent of local gradients (countergradient transport).

Bellingshausen Sea bathymetric compilation

The southern Bellingshausen Sea (SBS) is a rapidly-changing part of West Antarctica, where oceanic and atmospheric warming has led to the recent basal melting and break-up of the Wilkins ice shelf, the dynamic thinning of fringing glaciers, and sea-ice reduction. Accurate sea-floor morphology is vital for understanding the continued effects of each process upon changes within Antarctica's ice sheets. Here we present a new bathymetric grid for the SBS compiled from shipborne echo-sounder, spot-sounding and sub-ice measurements. The 1-km grid is the most detailed compilation for the SBS to-date, revealing large cross-shelf troughs, shallow banks, and deep inner-shelf basins that continue inland of coastal ice shelves. The troughs now serve as pathways which allow warm deep water to access the ice fronts in the SBS. Our dataset highlights areas still lacking bathymetric constraint, as well as regions for further investigation, including the likely routes of palaeo-ice streams. The new compilation is a major improvement upon previous grids and will be a key dataset for incorporating into simulations of ocean circulation, ice-sheet change and history. It will also serve forecasts of ice stability and future sea-level contributions from ice loss in West Antarctica, required for the next IPCC assessment report in 2013.

Description of recent sediments of Nile Delta, Red Sea and Gulf of Aden (Table 1-3)

The study of textural, structural, chemical, and physical properties of fine-grained recent marine sediments leads to the conclusion that only a few compositional factors are responsible for significant changes in mass physical characteristics in the upper meters below sea bottom. Fossil-induced porosity increases water content and liquid limit. It also seems to have partially influenced the plastic limit and plasticity index of calcareous sandy silts from the Red Sea and the western Gulf of Aden so that they become similar to the montmorillonite rich prodelta clays from the Nile Delta. Diagrams based on liquid limit and plasticity loose their original meaning in these cases. Activity of sediments rich in microorganisms can be higher than that of montmorillonitic clay. The shear strength-depth relationship of normally consolidated sediments is surprisingly little influenced by changes in sand or clay content and clay mineralogy. Only high lime content, submarine erosion and beginning cementation increase the strength considerably. Erosional disconformities near the present surface can be deduced from the strength-depth curve when as little as 1 or 2 m sediment have been removed. Flat or irregular strength-depth curves indicate beginning cementation and probably discontinuous sedimentation, provided the composition of the material remains in some degree constant. In our samples diagenetic pyrite, but no recristallisation of carbonates could be detected under the microscope. Underconsolidation and excess pore-water pressure, factors which tend to foster submarine slides, mud lumps, and diapiric folding, seem to be restricted Varito areas with mainly rapidly deposited, homogeneous or layered sediments. But where an abundance of burrowing organisms increases the vertical permeability of the sediment, normal consolidation and stable deposits are to be expected, at least in the upper meters below the present surface. According to 14C-determinations on calcareous microorganisms the rate of deposition of the investigated sediments seems to range from 26 to 167 cm per 1000 years.

Distribution of molluscs on the Atlantic continental shelf off Southern Spanish Sahara, West Africa

Two hundred and seventy five mollusc species from the continental shelf off Southern Spanish Sahara (depth: 32-60 m) were identified. Their distribution pattern is strongly influenced by the nature of the bottom (firm substrate, shelter, stability of sediment) rather than other factors at that depth interval. This faunal assemblage shows great affinity to the Mediterranean and Lusitanian faunas, and comprises only few (22 %) exclusively Senegalese and species living south of Senegal.

Investigation of microfossils on sediment core CRP-2 from the Ross Sea, Antarctica

Marine diatoms are the primary biostratigraphical and paleoenvironmental tool for interpreting the upper Palaeogene and lower Neogene strata recovered during the second drilling season of the Cape Roberts Project at site CRP-2 in the western Ross Sea, Antarctica. Silicoflagellates, ebridians, and a chrysophyte cyst provide supporting biostratigraphical information. More than 100 dominantly planktic diatom taxa are recognised. Of these, more than 30 are treated informally, pending SEM examination and formal description. Many other taxa are noted only to generic level. Lower Oligocene (c. 31 Ma) through lower Miocene (c. 18.5 Ma) diatoms occur from 28 mbsf down to 565 mbsf. Below this level, to the bottom of the hole at 624.15 mbsf, diatom assemblages are poorly-preserved and many samples are barren. A biostratigraphic zonal framework, consisting of ten diatom zones, is proposed for the Antarctic continental shelf. Ages inferred from the diatom biostratigraphy correspond well with geochronological data from argon dating of volcanic materials and strontium dating of calcareous macrofossils, as well as nannofossil biochronological datums. The biochronostratigraphical record from CRP-2/2A provides an important record of diatom events and mid-Cenozoic environmental changes in the Antarctic neritic zone.

Sea surface temperature calculation based on alkenones in sediments off northwest Africa

The molecular stratigraphy of Biogeochemical Oceanic Flux Study core 31K (19°N, 20°10'W) and Ocean Drilling Program Hole 658C (20°45'N, 18°35'W) has been studied for C37 alkenone abundances over the past 80 ka at high resolution (~circa 200-500 years). The derived Uk 37' sea surface temperature record for both cores shows a range of temperatures from about 18°C during the last glacial to 21.5°C during the early Holocene. Both records also reveal changes in sea surface temperature as much as 2°-4°C over a few hundred years, which correlate well with similar abrupt climatic changes observed in cores from elsewhere in the NE Atlantic, associated with 'Heinrich events'. Our results indicate that meltwater produced by these ice-rafting events was transmitted southward by the Canary Current, where it had considerable impact on sea surface temperatures in the subtropical eastern Atlantic.

(Table T1) Long-chain alkenones, UK'37 and derived sea surface temperatures of ODP Sites 184-1147 and 184-1148

A reconnaissance study of alkenone stratigraphy for the past 35 m.y. in the northern South China Sea (SCS) using sediments from Sites 1147 and 1148 of Ocean Drilling Program (ODP) Leg 184 has been completed. Alkenones were not detected in sediment samples older than ~31 Ma. However, C37:2 appeared in the sedimentary record between ~8 and 31 Ma and both C37:2 and C37:3 were present between 0 and 8 Ma. These changes in alkenone occurrences may signal a response to global-scale Neogene cooling as well as to monsoon intensification and sea level changes over time as a result of Himalayan uplift and the opening of the SCS. Alternatively, they may be related to an evolutionary record of the development of temperature control on alkenone production in coccolithophores. The Uk'37 index for 0-8 Ma produces sea-surface temperatures (SST) of 19°-26°C, which are in the range of previously determined glacial-interglacial values for the northern SCS. Before the late Pleistocene (~1.2 Ma), the SST range is between 23° and 26°C with less variation. This change in variability may signify the early stage of intensified winter monsoons where cold wind and waters from the north may not yet have had a significant effect on SST or it may be the evolutionary link between the early development of unsaturated alkenones in coccolithophores and modern temperature control of alkenone production. We believe a long-term alkenone record is useful for further understanding of global-scale neogene cooling, the development of the East Asian monsoon system, and the evolutionary development of temperature control on alkenone unsaturation. Our data indicate that a high-resolution Uk'37 record for at least the last ~8 Ma is feasible for the northern SCS.