Sea surface temperatures reconstructed from sediment core U1338

This study presents a new alkenone-derived Sea Surface Temperature (SST) record and d18ONoelaerhabdaceae data of the 2-5 µm carbonate fractions from the IODP site U1338 located in the Eastern Equatorial Pacific (EEP), over the Miocene-Pliocene. Our data and those available from other sites of the same area show the establishment of a cold tongue during the early Pliocene (4.4 - 3.6 Ma). SST and d18ONoelaerhabdaceae time-series indicate periods of significant salinity variations. Comparison with the d18Obenthic curve from sediment cores of the Equatorial Pacific Ocean allow us to distinguish between global changes and local surface salinity variations in the EEP. Ice sheet growth and evaporation-precipitation are then discussed as possible drivers of such changes, as well as the role of Central American and the Indonesian seaway restriction. Our data suggest a shallowing of the thermocline in the EEP, between 6.8 and 6 Ma, and its shoaling between 4.8 and 4.0 Ma, suggesting the appearance of the cold tongue (Steph et al., 2010). The Pliocene climate transition would therefore not be primarily driven by Northern hemisphere glaciation.

Age and alkenone-derived Holocene sea-surface temperature records of sediment core M40-4-SL78

This Special Issue of The Holocene contains 16 research papers based on a symposium at the 11th International Meeting of the European Union of Geosciences held in Strasbourg in April 2001. The aim of the symposium was a state-of-the-art assessment of empirical studies of postglacial marine and terrestrial climatic archives and their integration with numerical climate models. This editorial places the individual papers in the broader context of natural climate variability and anthropogenic impacts on the global climate system, regional differences in climate between maritime and continental areas, and the need for an improved theoretical basis for understanding the underlying causes of environmental change. The focus of the Special Issue is the dynamic and relatively well-understood climate of the North Atlantic and the European realm, where, in relation to the steepest offshore temperature gradient on Earth, observational data are abundant and many recent advances have been made in climate reconstruction from proxy archives. The editorial also contains a summary and overview of the papers included in the four main sections of the Special Issue, which emphasize: (1) numerical modelling experiments; (2) models of glacier buildup and equilibrium-line altitude; (3) marine and terrestrial proxy records of climatic change; and (4) multiproxy palaeoenvironmental reconstruction of a Portuguese lagoonal system.

Sea surface temperatures reconstructed from alkenones of surface sediment samples of the Argentine Basin at stations GeoB1501-1 to GeoB2830-1

We analysed the alkenone unsaturation ratio (UK'37) in 87 surface sediment samples from the western South Atlantic (5°N-50°S) in order to evaluate its applicability as a paleotemperature tool for this part of the ocean. The measured UK'37 ratios were converted into temperature using the global core-top calibration of Müller et al. (1998, doi:10.1016/S0016-7037(98)00097-0) and compared with annual mean atlas sea-surface temperatures (SSTs) of overlying surface waters. The results reveal a close correspondence (<1.5°C) between atlas and alkenone temperatures for the Western Tropical Atlantic and the Brazil Current region north of 32°S, but deviating low alkenone temperatures by -2° to -6°C are found in the regions of the Brazil-Malvinas Confluence (35-39°S) and the Malvinas Current (41-48°S). From the oceanographic evidence these low UK'37 values cannot be explained by preferential alkenone production below the mixed layer or during the cold season. Higher nutrient availability and algal growth rates are also unlikely causes. Instead, our results imply that lateral displacement of suspended particles and sediments, caused by strong surface and bottom currents, benthic storms, and downslope processes is responsible for the deviating UK'37 temperatures. In this way, particles and sediments carrying a cold water UK'37 signal of coastal or southern origin are transported northward and offshore into areas with warmer surface waters. In the northern Argentine Basin the depth between displaced and unaffected sediments appears to coincide with the boundary between the northward flowing Lower Circumpolar Deep Water (LCDW) and the southward flowing North Atlantic Deep Water (NADW) at about 4000 m.

Particle fluxes of the bathypelagic sediment trap CVOO-3 in the eastern tropical North Atlantic for the period December 2009 until May 2011

Particle fluxes at the Cape Verde Ocean Observatory (CVOO) in the eastern tropical North Atlantic for the period December 2009 until May 2011 are discussed based on bathypelagic sediment trap time-series data collected at 1290 and 3439 m water depth. The typically oligotrophic particle flux pattern with weak seasonality is modified by the appearance of a highly productive and low oxygen (minimum concentration below 2 µmol kg**-1 at 40 m depth) anticyclonic modewater eddy (ACME) in winter 2010. The eddy passage was accompanied by unusually high mass fluxes of up to 151 mg m**-2 d**-1, lasting from December 2009 to May 2010. Distinct biogenic silica (BSi) and organic carbon flux peaks of ~15 and 13.3 mg m**-2 d**-1, respectively, were observed in February-March 2010 when the eddy approached the CVOO. The flux of the lithogenic component, mostly mineral dust, was well correlated with that of organic carbon, in particular in the deep trap samples, suggesting a tight coupling. The lithogenic ballasting obviously resulted in high particle settling rates and, thus, a fast transfer of epi-/meso-pelagic signatures to the bathypelagic traps. We suspect that the two- to three-fold increase in particle fluxes with depth as well as the tight coupling of mineral dust and organic carbon in the deep trap samples might be explained by particle focusing processes within the deeper part of the eddy. Molar C : N ratios of organic matter during the ACME passage were around 18 and 25 for the upper and lower trap samples, respectively. This suggests that some productivity under nutrient (nitrate) limitation occurred in the euphotic zone of the eddy in the beginning of 2010 or that a local nitrogen recycling took place. The d15N record showed a decrease from 5.21 to 3.11 per mil from January to March 2010, while the organic carbon and nitrogen fluxes increased. The causes of enhanced sedimentation from the eddy in February/March 2010 remain elusive, but nutrient depletion and/or an increased availability of dust as a ballast mineral for organic-rich aggregates might have contributed. Rapid remineralisation of sinking organic-rich particles could have contributed to oxygen depletion at shallow depth. Although the eddy formed in the West African coastal area in summer 2009, no indications of coastal flux signatures (e.g. from diatoms) were found in the sediment trap samples, confirming the assumption that the suboxia developed within the eddy en route. However, we could not detect biomarkers indicative of the presence of anammox (anaerobic ammonia oxidation) bacteria or green sulfur bacteria thriving in photic zone suboxia/hypoxia, i.e. ladderane fatty acids and isorenieratene derivatives, respectively. This could indicate that suboxic conditions in the eddy had recently developed and/or the respective bacterial stocks had not yet reached detection thresholds. Another explanation is that the fast-sinking organic-rich particles produced in the surface layer did not interact with bacteria from the suboxic zone below. Carbonate fluxes dropped from -52 to 21.4 mg m**-2 d**-1 from January to February 2010, respectively, mainly due to reduced contribution of shallow-dwelling planktonic foraminifera and pteropods. The deep-dwelling foraminifera Globorotalia menardii, however, showed a major flux peak in February 2010, most probably due to the suboxia/hypoxia. The low oxygen conditions forced at least some zooplankton to reduce diel vertical migration. Reduced "flux feeding" by zooplankton in the epipelagic could have contributed to the enhanced fluxes of organic materials to the bathypelagic traps during the eddy passage. Further studies are required on eddy-induced particle production and preservation processes and particle focusing.