Sea surface temperature reconstruction for sediment core KNO5103-0029GGC

We present a 15 kyr sea surface temperature (SST) record for a high sedimentation rate core (KNR51-29GGC) from the Feni Drift off of Ireland, based on an organic geochemical technique for paleotemperature estimation, U37 K'. We compare the U37 K' temperature record to planktonic foraminiferal delta18O and foraminiferal assemblage SST estimates from the same sample horizons. U37 K' gives SST estimates of 13°C for the early deglacial and 18°C for the Holocene and Recent, whereas assemblages give estimates of 9°C and 13°C, respectively. As in nearby core V23-81, we find Ash Zone 1, the Younger Dryas increase in Neogloboquadrina pachyderma sinistral abundance, and maximum abundance of this species during glaciation. N. pachyderma dextral oxygen isotopic analyses have a late glacial to interglacial range of 1.5 per mil. A reduction of about 1 per mil in delta18O occurred at about 12 ka, whereas U37 K' and the foraminiferal fauna indicate a 2°C warming. This implies a 0.9 per mil salinity effect on delta18O which we attribute to meltwater freshening. All three parameters indicate cooling during the Younger Dryas. U37 K' SST estimates show that the major shift from deglacial to interglacial temperatures occurred after the Younger Dryas in termination 1b, in contrast to the assemblage data, which show this jump in SST at the end of the glaciation during termination Ia. Differences between the two SST estimators, which may result from their different (floral versus faunal) sources, are more pronounced between transitions Ia and Ib. This may reflect different habitats under the unusual sea surface conditions of the deglaciation.

Salinity estimation from Gulf of Guinea, sediment core MD03-2707

The isotopic composition of surface seawater is widely used to infer past changes in sea surface salinity using paired foraminiferal Mg/Ca and d18O from marine sediments. At low latitudes, paleosalinity reconstructions using this method have largely been used to document changes in the hydrological cycle. This method usually assumes that the modern seawater d18O (d18Osw)/salinity relationship remained constant through time. Modelling studies have shown that such assumptions may not be valid because large-scale atmospheric circulation patterns linked to global climate changes can alter the seawater d18Osw/salinity relationship locally. Such processes have not been evidenced by paleo-data so far because there is presently no way to reconstruct past changes in the seawater d18Osw/salinity relationship. We have addressed this issue by applying a multi-proxy salinity reconstruction from a marine sediment core collected in the Gulf of Guinea. We measured hydrogen isotopes in C37:2 alkenones (dDa) to estimate changes in seawater dD. We find a smooth, long-term increase of ~10 per mil in dDa between 10 and 3 kyr BP, followed by a rapid decrease of ~10 per mil in dDa between 3 kyr BP and core top to values slightly lighter than during the early Holocene. Those features are inconsistent with published salinity estimations based on d18Osw and foraminiferal Ba/Ca, as well as nearby continental rainfall history derived from pollen analysis. We combined dDa and d18Osw values to reconstruct a Holocene record of salinity and compared it to a Ba/Ca-derived salinity record from the same sedimentary sequence. This combined method provides salinity trends that are in better agreement with both the Ba/Ca-derived salinity and the regional precipitation changes as inferred from pollen records. Our results illustrate that changes in atmospheric circulation can trigger changes in precipitation isotopes in a counter-intuitive manner that ultimately impacts surface salinity estimates based on seawater isotopic values. Our data suggest that the trends in Holocene rainfall isotopic values at low latitudes may not uniquely result from changes in local precipitation associated with the amount effect.

Sea surface temperature reconstruction of ODP Site 175-1087

Successful application of the alkenone palaeothermometer, the UK'37 index, relies upon the assumption that fossil alkenone synthesisers responded to growth-temperature changes in a similar manner to the modern producers, chiefly the coccolithophores Emiliania huxleyi and Gephyrocapsa oceanica. We compare coccolith and UK'37 data from ODP Site 1087 in the south-east Atlantic between 1500 and 500 ka, and show that evolutionary events and changes in species dominance within the coccolithophore populations had little impact on the UK'37 record. The relative abundances of the C37 and C38 alkenones also closely resembled those found in modern populations, and suggest a similar temperature sensitivity of UK'37 during the early and mid-Pleistocene to that found at present. These results support the application of the UK'37 index to reconstruct sea-surface temperatures (SSTs) throughout the Quaternary. The UK'37 record at ODP Site 1087 contains an SST signal that documents the emergence of the 100-kyr cycles that characterise the late Quaternary ice volume records. This is preceded by significant cooling at ODP Site 1087, marked by a negative shift in SSTs and a positive shift in the planktonic delta18O some 250-kyr earlier, at ca 1150-1000 ka. This results in a permanent fall in average SSTs of around 1.5 °C. The predicted increase in aridity onshore as a result of this cooling can be identified in a number of published records from southern Africa, and may have played a role in some important evolutionary events of the mid-Pleistocene.

Sea surface temperature reconstruction from biomarker in sediments off Portugal

High resolution reconstructions of sea surface temperature (Uk'37-SST), coccolithophore associations and continental input (total organic carbon, higher plant n-alkanes, n-alkan-1-ols) in core D13882 from the shallow Tagus mud patch are compared to SST records from deep-sea core MD03-2699 and other western Iberian Margin cores. Results reveal millennial-scale climate variability over the last deglaciation, in particular during the LGIT. In the Iberian margin, Heinrich event 1 (H1) and the Younger Dryas (YD) represent two extreme episodes of cold sea surface condition separated by a marine warm phase that coincides with the Bølling-Allerød interval (B-A) on the neighboring continent. Following the YD event, an abrupt sea surface warming marks the beginning of the Holocene in this region. SSTs recorded in core D13882 changed, however, faster than those at deep-sea site MD03-2699 and at the other available palaeoclimate sequences from the region. While the SST values from most deep-sea cores reflect the latitudinal gradient detected in the Iberian Peninsula atmospheric temperature proxies during H1 and the B-A, the Tagus mud patch (core D13882) experienced colder SSTs during both events. This is most certainly related to a supplementary input of cold freshwater from the continent to the Tagus mud patch, a hypothesis supported by the high contents of terrigenous biomarkers and total organic carbon as well as by the dominance of tetra-unsaturated alkenone (C37:4) observed at this site. The comparison of all western Iberia SST records suggests that the SST increase that characterizes the B-A event in this region started 1000 yr before meltwater pulse 1A (mwp-1A) and reached its maximum values during or slightly after this episode of substantial sea-level rise. In contrast, during the YD/ Holocene transition, the sharp SST rise in the Tagus mud patch is synchronous with meltwater pulse IB. The decrease of continental input to the mud patch conflrms a sea level rise in the region. Thus, the synchronism between the maximum warming in the mid-latitudes off the western Iberian margin, the adjacent landmasses and Greenland indicates that mwp-lB and the associated sea-level rise probably initiated in the Northern Hemisphere rather than in the South.

Stable carbon isotopic composition of biomarker lipids of DSDP Hole 31-302

The middle Paleocene through early Eocene long-term gradual warming was superimposed by several transient warming events, such as the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum 2 (ETM2). Both events show evidence for extreme global warming associated with a major injection of carbon into the ocean-atmosphere system, but the mechanisms of carbon injection and many aspects of the environmental response are still poorly understood. In this study, we analyzed the concentration and stable carbon isotopic (d13C) composition of several sulfur-bound biomarkers derived from marine photoautotrophs, deposited in the Arctic Ocean at ~85°N, during ETM2. The presence of sulfur-bound biomarkers across this event points toward high primary productivity and anoxic bottom water conditions. The previously reported presence of isorenieratene derivatives indicates euxinic conditions in the photic zone, likely caused by a combination of enhanced primary productivity and salinity stratification. The negative carbon isotope excursion measured at the onset of ETM2 for several biomarkers, ranges between 3 per mil and 4.5 per mil, much larger than the ~1.4 per mil recorded in marine carbonates elsewhere, suggesting substantial enhanced isotopic fractionation by the primary producers likely due to a significant rise in pCO2. In the absence of biogenic carbonates in the ETM2 section of our core we use coeval planktonic d13C from elsewhere to estimate surface water d13C in the Arctic Ocean and then apply the relation between isotopic fractionation and pCO2, originally calibrated for haptophyte alkenones, to three selected organic biomarkers (i.e., S-bound phytane, C35 hopane, and a C25 highly branched isoprenoid). This yields pCO2 values potentially in the range of four times preindustrial levels. However, these estimates are uncertain because of a lack of knowledge on the importance of pCO2 on photosynthetic isotopic fractionation.

Carbon contents of lower Cretaceous sediments from ODP Hole 198-1213B (Table 1)

ODP Leg 198 drilling on Shatsky Rise recovered a lower Aptian porcellanite (~120.5 Ma) deposited during oceanic anoxic event (OAE) 1a that contains C36-C39 alkadienones: C37:2 and C39:2 alkadien-2-ones and C36:2 and C38:2 alkadien-3-ones. This alkenone distribution differs from that typical of contemporary sediments and haptophyte algae, but resembles that of Cretaceous sediments from the Blake-Bahama basin. The discovery of alkenones in the early Aptian extends their sedimentary record by 15 M.y. to 120.5 M.y. and demonstrates the potential for long-term survival of these diagnostic functional lipids under favorable depositional conditions and subsequent shallow burial. It also contributes to the understanding and reconstruction of evolutionary developments in alkenone distributions and biosynthesis over geologic time.

Biomarker and radiogenic isotopes from sediment core POS362-2_33

The termination of the African Humid Period in northeastern Africa during the early Holocene was marked by the southward migration of the rain belt and the disappearance of the Green Sahara. This interval of drastic environmental changes was also marked by the initiation of food production by North African huntergatherer populations and thus provides critical information on human-environment relationships. However, existing records of regional climatic and environmental changes exhibit large differences in timing and modes of the wet/dry transition at the end of the African Humid Period. Here we present independent records of changes in river runoff, vegetation and erosion in the Nile River watershed during the Holocene obtained from a unique sedimentary sequence on the Nile River fan using organic and inorganic proxy data. This high-resolution reconstruction allows to examine the phase relationship between the changes of these three parameters and provides a detailed picture of the environmental conditions during the Paleolithic/Neolithic transition. The data show that river runoff decreased gradually during the wet/arid transition at the end of the AHP whereas rapid shifts of vegetation and erosion occurred earlier between 8.7 and about 6 ka BP. These asynchronous changes are compared to other regional records and provide new insights into the threshold responses of the environment to climatic changes. Our record demonstrates that the degradation of the environment in northeastern Africa was more abrupt and occurred earlier than previously thought and may have accelerated the process of domestication in order to secure sustainable food resources for the Neolithic African populations.

Sea surface temperature estimation of the South Tasman Rise

The Subtropical Front (STF) marking the northern boundary of the Southern Ocean has a steep gradient in sea surface temperature (SST) of approximately 4°C over 0.5° of latitude. Presently, in the region south of Tasmania, the STF lies nominally at 47°S in the summer and 45°S in the winter. We present here SST reconstructions in a latitudinal transect of cores across the South Tasman Rise, southeast of Australia, during the late Quaternary. SST reconstructions are based on two paleotemperature proxies, alkenones and faunal assemblages, which are used to assess past changes in SST in spring and summer. The north-south alignment in core locations allows reconstruction of movement of the STF over the last 100 ka. Surface water temperatures during the last glaciation in this region were ~4°C colder than today. Additional temperature changes greater in magnitude than 4°C seen in individual cores can be attributed to changes in the water mass overlying the core site caused by the movement of the front across that location. During the penultimate interglacial, SST was ~2°C warmer and the STF was largely positioned south of 47°S. Movement of the STF to the north occurred during cool climate periods such as the last marine isotope stages 3 and 4. In the last glaciation, the front was at its farthest north position, becoming pinned against the Tasmanian landmass. It moved south by 4° latitude to 47°S in summer during the deglaciation but remained north of 45°S in spring throughout the early deglaciation. After 11 ka B.P. inferred invigoration of the East Australia Current appears to have pushed the STF seasonally south of the East Tasman Plateau, until after 6 ka B.P. when it achieved its present configuration.

Paleo-sea surface temperature calculations in the equatorial east Atlantic

We present two ~270 kyr paleo-sea surface temperature (SST) records from the Equatorial Divergence and the South Equatorial Current derived from Mg/Ca ratios in the planktic foraminifer Globigerinoides sacculifer. The present study suggests that the magnesium signature of G. sacculifer provides a seasonal SST estimate from the upper ~50 m of the water column generated during upwelling in austral low-latitude fall/winter. Common to both down-core records is a glacial-interglacial amplitude of ~3°-3.5°C for the last climatic changes and lower Holocene and glacial oxygen isotope stage 2 temperatures compared with interglacial stage 5.5 and glacial stage 6 temperatures, respectively. The comparison to published SST estimates from alkenones, oxygen isotopes, and foraminiferal transfer function from the same core material pinpoints discrepancies and conformities between methods.

Paleocoenographic investigations on sediment profile GeoB6008

Near-shore waters along the northwest African margin are characterized by coastal upwelling and represent one of the world's major upwelling regions. Sea surface temperature (SST) records from Moroccan sediment cores, extending back 2500 years, reveal anomalous and unprecedented cooling during the 20th century, which is consistent with increased upwelling. Upwelling-driven SSTs also vary out of phase with millennial-scale changes in Northern Hemisphere temperature anomalies (NHTAs) and show relatively warm conditions during the Little Ice Age and relatively cool conditions during the Medieval Warm Period. Together, these results suggest that coastal upwelling varies with NHTAs and that upwelling off northwest Africa may continue to intensify as global warming and atmospheric CO2 levels increase.