TEX86H proxy and BIT index of sediment core GeoB9528-3 and reconstructed temperature data from NW Africa surface sediments

Changes in the strength of Atlantic meridional overturning circulation (AMOC) are known to have profound impacts on global climate. Coupled modelling studies have suggested that, on annual to multi-decadal time scales, a slowdown of AMOC causes a deepening of the thermocline in the tropical Atlantic. However, this process has been poorly constrained by sedimentary geochemical records. Here, we reconstruct surface (UK'37 Index) and thermocline (TEX86H) water temperatures from the Guinea Plateau Margin (Eastern tropical Atlantic) over the last two glacial-interglacial cycles (~ 192 kyr). These paleotemperature records show that periods of reduced AMOC, as indicated by the d13 C benthic foraminiferal record from the same core, coincide with a reduction in the near-surface vertical temperature gradient, demonstrating for the first time that AMOC-induced tropical Atlantic thermocline adjustment exists on longer, millennial time scales. Modelling results support the interpretation of the geochemical records and show that thermocline adjustment is particularly pronounced in the eastern tropical Atlantic. Thus, variations in AMOC strength appear to be an important driver of the thermocline structure in the tropical Atlantic from annual to multi-millennial time scales.

Radiolarian-based paleotemperatures of ODP Site 177-1089 in the Atlantic Sector of the Southern Ocean

Two cores, Site 1089 (ODP Leg 177) and PS2821-1, recovered from the same location (40°56'S; 9°54'E) at the Subtropical Front (STF) in the Atlantic Sector of the Southern Ocean, provide a high-resolution climatic record, with an average temporal resolution of less than 600 yr. A multi-proxy approach was used to produce an age model for Core PS2821-1, and to correlate the two cores. Both cores document the last climatic cycle, from Marine Isotopic Stage 6 (MIS 6, ca. 160 kyr BP, ka) to present. Summer sea-surface temperatures (SSSTs) have been estimated, with a standard error of ca. +/-1.16°C, for the down core record by using Q-mode factor analysis (Imbrie and Kipp method). The paleotemperatures show a 7°C warming at Termination II (last interglacial, transition from MIS 6 to MIS 5). This transition from glacial to interglacial paleotemperatures (with maximum temperatures ca. 3°C warmer than present at the core location) occurs earlier than the corresponding shift in delta18O values for benthic foraminifera from the same core; this suggests a lead of Southern Ocean paleotemperature changes compared to the global ice-volume changes, as indicated by the benthic isotopic record. The climatic evolution of the record continues with a progressive temperature deterioration towards MIS 2. High-frequency, millennial-scale climatic instability has been documented for MIS 3 and part of MIS 4, with sudden temperature variations of almost the same magnitude as those observed at the transitions between glacial and interglacial times. These changes occur during the same time interval as the Dansgaard-Oeschger cycles recognized in the delta18Oice record of the GRIP and GISP ice cores from Greenland, and seem to be connected to rapid changes in the STF position in relation to the core location. Sudden cooling episodes ('Younger Dryas (YD)-type' and 'Antarctic Cold Reversal (ACR)-type' of events) have been recognized for both Termination I (ACR-I and YD-I events) and II (ACR-II and YD-II events), and imply that our core is located in an optimal position in order to record events triggered by phenomena occurring in both hemispheres. Spectral analysis of our SSST record displays strong analogies, particularly for high, sub-orbital frequencies, to equivalent records from Vostok (Antarctica) and from the Subtropical North Atlantic ocean. This implies that the climatic variability of widely separated areas (the Antarctic continent, the Subtropical North Atlantic, and the Subantarctic South Atlantic) can be strongly coupled and co-varying at millennial time scales (a few to 10-ka periods), and eventually induced by the same triggering mechanisms. Climatic variability has also been documented for supposedly warm and stable interglacial intervals (MIS 1 and 5), with several cold events which can be correlated to other Southern Ocean and North Atlantic sediment records.

(Figure 1) Silicoflagellates at DSDP Hole 69-504

Diverse and abundant late Miocene to Pleistocene silicoflagellates at DSDP Site 504 can be correlated by tropical biostratigraphic zones and relative paleotemperature values to eastern tropical Pacific reference site DSDP 503A farther to the west. Early Pliocene assemblages, which were poorly known until now, are present and can be correlated locally between DSDP Holes 504, 503A, and 495, using species events associated with the new Dictyocha pulchella Subzone and Dictyocha angulata Subzone. Silicoflagellate relative paleotemperature values show major warming at 4.7 to 5.0 Ma (Cores 45-48), 3.4 to 3.8 Ma (Cores 32-33), 1.5 to 1.7 Ma (Cores 12-16), and 0.5 to 0.8 Ma (Cores 3-6). Major coolings occurred at 5.0 to 5.1 Ma (Core 51), 3.9 to 4.4 Ma (Cores 38-44), and 1.0 to 1.3 Ma (Cores 8-10). The appearance of Dictyocha longa is proposed to replace the asperoid/fibuloid ratio reversal as the bottom of the Dictyocha fibula Zone, because the non-evolutionary ratio reverses several times in the upper Miocene of Hole 503A, and at least once in Hole 504. Three new Pliocene silicoflagellates are defined: Dictyocha concinna Bukry, n. sp., D. helix Bukry, n. sp., and D. tamarae Bukry, n. sp.

(Table 1) Age determination of North Atlantic sediment cores

Four cores raised from the eastern Norwegian Sea and adjacent Norwegian fjords at sites influenced by Atlantic water have been investigated. Oxygen isotope analyses in benthic and planktonic foraminifera are used as a proxy for the paleotemperature development spanning the last 800 years. The cores have been dated using a combination of 210Pb and radiocarbon dates yielding time resolutions of 2-5 years for the last century and 9-25 years beyond this. The proxy records have been compared with instrumental time series covering the last 100 years in order to validate the oxygen isotope measurements as a proxy for paleotemperature. The comparison shows that the paleotemperature variability derived from the oxygen isotope analyses is generally similar to the amplitudes and trends seen in the instrumental time series. In particular, a cooling around 1905-1925 followed by a warming until 1955 is evident in all proxy records as well as in the instrumental time series. Beyond the last century the proxy records show two periods from ~1225-1450 and ~1650-1905(25) when temperatures were 1.3-1.6°C lower than present separated by a period of temperatures periodically comparable to present. The last 80 years represent the modern warming and appear to be the warmest period of the last 800 years. We find that that the ocean temperature variability is comparable to terrestrial reconstructions from the region implying a strong link in the ocean-atmosphere climate system. This suggests that the climate variability in this region beyond the period covered by instrumental time series was also associated with changes in the thermohaline circulation.