Planktonic foraminiferal d18O and d13C values, derived sea-surface temperatures from d18O and selected planktonic foraminiferal species for core P69 of Hawke Bay off eastern North Island

Recent evidence suggests that the Subtropical Convergence (STC) zone east of New Zealand shifted little from its modern position along Chatham Rise during the last glaciation, and that offshore surface waters north of the STC zone cooled only slightly. However, at nearshore core site P69 (2195 m depth), 115 km off the east coast of North Island and ca 300 km north of the modern STC zone, planktonic foraminiferal species, transfer function data and stable oxygen and carbon isotope records suggest that surface waters were colder by up to 6°C during the late last glacial period compared to the Holocene, and included a strong upwelling signature. Presently site P69 is bathed by south-flowing subtropical waters in the East Cape Current. The nearshore western end of Chatham Rise supports a major bathymetric depression, the Mernoo Saddle, through which some exchange between northern subtropical and southern subantarctic water presently occurs. It is proposed that as a result of much intensified current flows south of the Rise during the last glaciation, a consequence of more compressed subantarctic water masses, lowered sea level, and an expanded and stronger Westerly Wind system, there was accelerated leakage northwards of both Australasian Subantarctic Water and upwelled Antarctic Intermediate Water over Mernoo Saddle in a modified and intensified Southland Current. The expanded cold water masses displaced the south-flowing warm East Cape Current off southeastern North Island, and offshore divergence was accompanied by wind-assisted upwelling of nutrient-rich waters in the vicinity of P69. A comparable kind of inshore cold water jetting possibly characterised most glacial periods since the latest Miocene, and may account for the occasional occurrence of subantarctic marine fossils in onland late Cenozoic deposits north of the STC zone, rather than invoking wholesale major oscillations of the oceanic STC itself.

(Table 3) Mediterranean core-top d18O values for various planktonic foraminiferal species

A box model is presented to simulate changes in Mediterranean long-term average salinity and d18O over the past 20,000 years. Simulations are validated by comparison with observations. Sensitivity tests illustrate robustness with respect to the main assumptions and uncertainties. The results show that relative humidity over the Mediterranean remained relatively constant around 70%, apparently narrowly constrained to the lower end of the range observed globally over sea surfaces by the basin's land-locked character. Isotopic depletion in run off, relative to the present, is identified as the main potential cause of depletions in the Mediterranean d18O record. Also, slight increases in relative humidity (of the order of 5%) might have caused very pronounced isotopic depletions, such as that in sapropel S5 of the penultimate interglacial maximum. The model shows distinctly non proportional responses of d18O and salinity to environmental change, which argues against the use of isotope residuals in Mediterranean paleosalinity reconstructions.

(Table 1) Core top Globorotalia truncatulinoides d18O ratios of Atlantic Ocean samples

We investigate whether the oxygen isotope ratio in the test of Globorotalia truncatulinoides can serve as a proxy for intermediate depth (200-500 m) density. Since intermediate depth horizontal density gradients are associated with the vertical shear of upper ocean flows, this proxy could provide a tool for reconstructing past ocean circulation. The spatial pattern of core top Gr. truncatulinoides d18O in the Atlantic Ocean mimics the upper ocean density gradients associated with the major ocean currents. To better constrain the controls on the calcification depth(s) of Gr. truncatulinoides, we attempt to simulate the surface sediment data set using water column temperature and salinity conditions above the core sites. We predicted foraminiferal d18O for each core site assuming (1) the calcification occurs at a single depth and (2) the initial calcification is at the surface and the subsequent calcification is at 800 m water depth. The predicted d18O best resembles measured d18O of Gr. truncatulinoides when using (1) a single depth calcification at 350 m or (2) a two-depth approximation with 30% surface and 70% 800-m calcification. This result gives us confidence in the ability of d18O in Gr. truncatulinoides to proxy lateral density gradients at the intermediate depths associated with upper ocean flow.

Multi-proxy record of planktonic and benthic foraminiferal d18O, and Mg/Ca-derived surface and lower thermocline temperatures in sediment core SO18471 (GIK18471), Timor Sea

Paleostudies of the Indonesian Throughflow (ITF) are largely based on temperature and salinity reconstructions of its near surface component, whereas the variability of its lower thermocline flow has rarely been investigated. We present a multi-proxy record of planktonic and benthic foraminiferal d18O, Mg/Ca-derived surface and lower thermocline temperatures, X-ray fluorescence (XRF)-derived runoff and sediment winnowing for the past 130 ka in marine sediment core SO18471. Core SO18471, retrieved from a water depth of 485 m at the southern edge of the Timor Strait close to the Sahul Shelf, sits in a strategic position to reconstruct variations in both the ITF surface and lower thermocline flow as well as to investigate hydrological changes related to monsoon variability and shelf dynamics over time. Sediment winnowing demonstrates that the ITF thermocline flow intensified during MIS 5d-a and MIS 1. In contrast during MIS 5e, winnowing was reduced and terrigenous input increased suggesting intensification of the local wet monsoon and a weaker ITF. Lower thermocline warming during globally cold periods (MIS 4 - MIS 2) appears to be related to a weaker and contracted thermocline ITF and advection of warm and salty Indian Ocean waters.

Dinocyst-derived sea-surface characteristics, palynomorph abundances, ice-rafted debris, and planktonic foraminifera abundances and d18O of the 35-41 ka BP interval of core MD99-2285

The last glacial millennial climatic events (i.e. Dansgaard-Oeschger and Heinrich events) constitute outstanding case studies of coupled atmosphere-ocean-cryosphere interactions. Here, we investigate the evolution of sea-surface and subsurface conditions, in terms of temperature, salinity and sea ice cover, at very high-resolution (mean resolution between 55 and 155 years depending on proxies) during the 35-41 ka cal BP interval covering three Dansgaard-Oeschger cycles and including Heinrich event 4, in a new unpublished marine record, i.e. the MD99-2285 core (62.69°N; -3.57s°E). We use a large panel of complementary tools, which notably includes dinocyst-derived sea-ice cover duration quantifications. The high temporal resolution and multiproxy approach of this work allows us to identify the sequence of processes and to assess ocean-cryosphere interactions occurring during these periodic ice-sheet collapse events. Our results evidence a paradoxical hydrological scheme where (i) Greenland interstadials are marked by a homogeneous and cold upper water column, with intensive winter sea ice formation and summer sea ice melting, and (ii) Greenland and Heinrich stadials are characterized by a very warm and low saline surface layer with iceberg calving and reduced sea ice formation, separated by a strong halocline from a less warm and saltier subsurface layer. Our work also suggests that this stadial surface/subsurface warming started before massive iceberg release, in relation with warm Atlantic water advection. These findings thus support the theory that upper ocean warming might have triggered European ice-sheet destabilization. Besides, previous paleoceanographic studies conducted along the Atlantic inflow pathways close to the edge of European ice-sheets suggest that such a feature might have occurred in this whole area. Nonetheless, additional high resolution paleoreconstructions are required to confirm such a regional scheme.

Dahlak coral d18O record (DGII) from the southern Red Sea

Coral palaeoclimatic studies are under way at many sites throughout the wet tropics. However, arid environments have received less attention. Here we report a high-resolution, 63 yr record of coral d18O and d13C extracted from a Porites colony from the Dahlak Archipelago, off the Eritrean coast, in the southern Red Sea. The annual cycles of the coral d18O and d13C are inversely related while their inter-annual variations show a strong positive correlation, with similar inter-decadal trends. Inter-annual variations in coral d18O show a relatively weak correlation with the southern Red Sea SST, but are strongly correlated with the Indian Ocean SST, especially on the decadal time-scale. The range of the inter-annual variations in the coral d18O is high compared to changes in local SST, due to the amplifying effect of simultaneous changes in water isotopic composition. Due to this amplification of the climate signal the coral provides a better indication of regional oceangraphic behaviour than the local SST record. The norrtheast monsoon signal in the coral d18O dominates the mean annual signal and shows the best correlation with the instrumental data sets. It appears that variations in the coral d18O are controlled mainly by variations in the intensity of surface water influx from the Indian Ocean to the Red Sea during the winter northeast monsoon. Of particular significance is that the decadal time-scale variations in the coral skeletal d18O are closely correlated with both the Indian Ocean SST and with variations in the Pacific-based Southern Oscillation index. That is, isotopically light coral skeleton, indicating strong NE monsoon Red Sea inflow, correlates with periods of high Indian Ocean SST and with predominantly negative (El Nino) phases of the Southern Oscillation. The simultaneous nature of inter-decadal changes in Asian monsoon and ENSO behaviour suggest pan-Indo-Pacific tropical climate reorganisation and evolution.

Age and d18O analysis relating to Cenozoic greenhouse climatesXXX

Past episodes of greenhouse warming provide insight into the coupling of climate and the carbon cycle and thus may help to predict the consequences of unabated carbon emissions in the future.