Description: The sea surface temperature (SST) of the tropical Indian Ocean is a major component of global climate teleconnections. While the Holocene SST history is documented for regions affected by the Indian and Arabian monsoons, data from the near-equatorial western Indian Ocean are sparse. Reconstructing past zonal and meridional SST gradients requires additional information on past temperatures from the western boundary current region. We present a unique record of Holocene SST and thermocline depth variations in the tropical western Indian Ocean as documented in foraminiferal Mg/Ca ratios and d18O from a sediment core off northern Tanzania. For Mg/Ca and thermocline d18O, most variance is concentrated in the centennial to bicentennial periodicity band. On the millennial time scale, an early to mid-Holocene (~7.8-5.6 ka) warm phase is followed by a temperature drop by up to 2°C, leading to a mid-Holocene cool interval (5.6-4.2 ka). The shift is accompanied by an initial reduction in the difference between surface and thermocline foraminiferal d18O, consistent with the thickening of the mixed layer and suggestions of a strengthened Walker circulation. However, we cannot confirm the expected enhanced zonal SST gradient, as the cooling of similar magnitude had previously been found in SSTs from the upwelling region off Sumatra and in Flores air temperatures. The SST pattern probably reflects the tropical Indian Ocean expression of a large-scale climate anomaly rather than a positive Indian Ocean Dipole-like mean state.

Description: We analysed Mg/Ca, Sr/Ca and Ca isotope ratios of benthonic foraminifers from sediment core tops retrieved during several research cruises in the Atlantic Ocean, in order to improve the understanding of isotope fractionation and element partitioning resulting from biomineralisation processes and changes in ambient conditions. Species include foraminifers secreting tests composed of hyaline low magnesium calcite, porcelaneous high magnesium calcite as well as aragonite. Our results demonstrate systematic isotope fractionation and element partitioning patterns specific for these foraminiferal groups. Calcium isotope fractionation is similar in porcelaneous and hyaline calcite tests and both groups demonstrate the previously described anomaly with enrichment of heavy isotopes around 3 - 4 °C (Gussone and Filipsson, 2010). Calcium isotope ratios of the aragonitic species Hoeglundina elegans, on the other hand, are about 0.4 per mil lighter compared to the calcitic species, which is in general agreement with stronger fractionation in inorganic aragonite compared to calcite. However, the low and strongly variable Sr content suggests additional processes during test formation, and we propose that transmembrane ion transport or a precursor phase to aragonite may be involved. Porcelaneous tests, composed of high Mg calcite, incorporate higher amounts of Sr compared to hyaline low Mg calcite, in agreement with inorganic calcite systematics, but also porcelaneous tests with reduced Mg/Ca show high Sr/Ca. While calcium isotopes, Sr/Ca and Mg/Ca in benthonic foraminifers primarily appear to fractionate and partition with a dominant inorganic control, d44/40Ca temperature and growth rate dependencies of benthonic foraminifer tests favour a dominant contribution of light Ca by transmembrane transport relative to unfractionated seawater Ca to the calcifying fluid, thus controlling the formation of foraminiferal d44/40Ca and Sr/Ca proxy signals.

Description: The temporal succession of changes in Amazonian hydroclimate during Heinrich Stadial 1 (HS1) (ca. 18-14.7 cal ka BP) is currently poorly resolved. Here we present HS1 records based on isotope, inorganic and organic geochemistry from a marine sediment core influenced by the Amazon River discharge. Our records offer a detailed reconstruction of the changes in Amazonian hydroclimate during HS1, integrated over the basin. We reconstructed surface water hydrography using stable oxygen isotopes (d18O) and Mg/Ca-derived paleotemperatures from the planktonic foraminifera Globigerinoides ruber, as well as salinity changes based on stable hydrogen isotope (dD) of palmitic acid. We also analyzed branched and isoprenoid tetraether concentrations, and compared them to existing bulk sediment ln(Fe/Ca) data and vegetation reconstruction based on stable carbon isotopes from n-alkanes, in order to understand the relationship between continental precipitation, vegetation and sediment production. Our results indicate a two-phased HS1 (HS1a and HS1b). During HS1a (18-16.9 cal ka BP), a first sudden increase of sea surface temperatures (SST) in the western equatorial Atlantic correlated with the slowdown of the Atlantic Meridional Overturning Circulation (AMOC) and the associated southern hemisphere warming phase of the bipolar seesaw. This phase was also characterized by an increased delivery of terrestrial material. During HS1b (16.9-14.8 cal ka BP), a decrease in terrestrial input was, however, associated with a marked decline of seawater d18O and palmitic acid dD. Both isotopic proxies independently indicate a drop in sea surface salinity (SSS). A number of records under the influence of the North Brazil Current, in contrast, indicate increases in SST and SSS resulting from a weakened AMOC during HS1. Our records thus suggest that the expected increase in SSS due to the AMOC slowdown was overridden by a two-phased positive precipitation anomaly in Amazonian hydroclimate.

Description: The western tropical Atlantic plays an important role in the interhemispheric redistribution of heat during millennial-scale changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC). The proper evaluation of this role depends on a clear understanding of sea surface temperature (SST) variations during AMOC slowdown periods like Heinrich Stadials (HS) in the western tropical Atlantic. However, published SST records from the western tropical Atlantic between ca. 4°S and 7°N show inconsistencies that are apparently related to the employed temperature proxy (i.e., Mg/Ca versus alkenone unsaturation index U37k′). In general, while Mg/Ca values indicate warming during Heinrich Stadials, U37k′ values show cooling. To assess this issue, we sampled core GeoB16224-1 retrieved off French Guiana (i.e., 6°39.38′N) and reconstructed water temperatures at high resolution using Mg/Ca on the foraminifera species Globigerinoides ruber, U37k′, TEX86 and modern analogue technique (MAT) transfer functions using planktonic foraminifera assemblages calibrated for 50 m water depth. Our results show that Mg/Ca and TEX86 values recorded an increase in SST related to AMOC slowdown. Conversely, U37k′ and MAT values registered a decrease in temperatures during HS3 and HS1. Our U37k′ and Mg/Ca results thus confirm the previously reported inconsistency for the period between 48-13 cal ka BP. We suggest that several non-thermal physiological effects probably imparted a negative temperature bias on the U37k′ temperatures during Heinrich Stadials. However, MAT-based temperatures show similar variability with U37k′-based temperatures. Hence, we also suggest that during severe slowdown periods of the AMOC, a steeper meridional temperature gradient together with a southward shift of the Intertropical Convergent Zone produced not only an increase in SST but also a stronger upper water column stratification and a shoaling of the thermocline, decreasing subsurface temperatures. Our new high resolution temperature records allow a better characterization of the thermal response of the upper water column in the tropical western Atlantic to slowdown events of the AMOC, reconciling previously discrepant records.

Description: Instrumental climate observations provide robust records of global land and ocean temperatures during the twentieth century. Unlike for temperature, continuous salinity observations in the surface ocean are scarce prior to 1970, and the magnitude of salinity changes during the twentieth century is largely unknown. Surface ocean salinity is a major component in climate dynamics, as it influences ocean circulation and water mass formation. Here we present an annually resolved reconstruction of salinity variations in the surface waters of the western subtropical North Pacific Ocean since 1873, based on bimonthly records of d18O, Sr/Ca, and U/Ca in a coral from the Ogasawara Islands. The reconstruction indicates that an abrupt regime shift toward fresher surface ocean conditions occurred between 1905 and 1910. Observational atmospheric data suggest that the abrupt freshening was associated with a weakening of the winds that drive the Kuroshio Current system and the associated subtropical gyre circulation. We note that the abrupt early-twentieth-century freshening in the western subtropical North Pacific precedes abrupt climate change in the northern North Atlantic by a few years. The potential for abrupt regime shifts in surface ocean salinity should be considered in climate predictions for the coming decades.

Description: Records of skeletal delta18O in monthly and Sr/Ca ratios in half-yearly resolution were obtained from a Bermuda coral (Diploria labyrinthiformis) for the time period 1520-1603 (+/-15 yr) AD within the Little Ice Age. Annual and decadal averages of both sea-surface temperature proxies indicate temperature variabilities of 0.5°C (standard deviation) and 0.3°C, respectively. Both numbers are close to recent instrumental observations. Approximately 30% of the interannual time series variance of delta18O is concentrated in broad bands centered at periods of ~30, 16, and 7.8 yr, the last two reflecting the influence of the North Atlantic Oscillation. Although this large-scale climate signal is present in the record, there is no correlation with other contemporaneous northern hemisphere proxy data, resulting from spatial differences in climate variability.

Description: The last interglacial period (about 125,000 years ago) is thought to have been at least as warm as the present climate (Kukla et al., 2002, doi:10.1006/qres.2001.2316). Owing to changes in the Earth's orbit around the Sun, it is thought that insolation in the Northern Hemisphere varied more strongly than today on seasonal timescales (Berger, 1987, doi:10.1175/1520-0469(1978)035〈2362:LTVODI〉2.0.CO;2), which would have led to corresponding changes in the seasonal temperature cycle (Montoya et al., 2000, doi:10.1175/1520-0442(2000)013〈1057:CSFKBW〉2.0.CO;2). Here we present seasonally resolved proxy records using corals from the northernmost Red Sea, which record climate during the last interglacial period, the late Holocene epoch and the present. We find an increased seasonality in the temperature recorded in the last interglacial coral. Today, climate in the northern Red Sea is sensitive to the North Atlantic Oscillation (Felis et al., 2000 doi:10.1029/1999PA000477; Rimbu et al., 2001, doi:10.1029/2001GL013083), a climate oscillation that strongly influences winter temperatures and precipitation in the North Atlantic region. From our coral records and simulations with a coupled atmosphere-ocean circulation model, we conclude that a tendency towards the high-index state of the North Atlantic Oscillation during the last interglacial period, which is consistent with European proxy records (Zagwijn, 1996, doi:10.1016/0277-3791(96)00011-X; Aalbersberg and Litt, 1998, doi:10.1002/(SICI)1099-1417(1998090)13:5〈367::AID-JQS400〉3.0.CO;2-I; Klotz et al., 2003, doi:10.1016/S0921-8181(02)00222-9), contributed to the larger amplitude of the seasonal cycle in the Middle East.

Description: Paleoceanographic reconstructions from the Brazil Current are scarce and lack the required temporal resolution to appropriately represent its variability during key periods of the last glacial-interglacial cycles. Here, we present the first high-temporal resolution multiproxy reconstruction of the Brazil Current at 24 °S covering the last 185 ka. During the last and penultimate glacial periods, our Mg/Ca-derived sea surface temperature (SST) record shows a strong cooling at ca. 47 and ca. 156 ka, respectively, that is followed by a warming trend from late-Marine Isotope Stage (MIS) 3 to MIS 1 and from late-MIS 6 to MIS5e, respectively. Importantly, the Brazil Current warmed uninterruptedly towards Termination I (II) after the low SST at ca. 47 and ca. 156 ka, with no SST minima during the Last Glacial Maximum or penultimate glacial maximum. The reason for the strong cooling and the warming trend during late-MIS 3 and late-MIS 6 could reside in the favorable obliquity configuration. However, this mechanism is not sufficient to sustain the warming observed for the rest of the last and penultimate glacial periods. We propose that the change in the Atlantic meridional overturning circulation (AMOC), as described in the literature, from a 'warm' to a 'cold mode' for MIS 2 and MIS 6 is responsible for the accumulation of warm waters in the subtropical western South Atlantic, preventing SST minima during the last and penultimate glacial maxima in the region. Change in benthic d13C corroborates that a fundamental modification in the AMOC mode might have triggered the heat accumulation. Our data also show a sudden increase in SST and surface salinity during the last glacial descent (MIS 4), indicating that the western portion of the subtropical gyres may have acted as a heat and salt reservoir, while higher latitude climates transited to a glacial background. Our findings imply that the AMOC 'cold mode' induces heat storage in the subtropical western South Atlantic and, because of that, the last two regional SST minima occurred out-of-phase with the glacial maxima of higher latitudes.

Description: Details regarding seawater temperature and isotopic composition of seawater in the Central Paratethys, during the Miocene epoch, are not well known. Here we present an insight based on results from a combination of Mg/Ca, δ18O and δ13C on foraminifera from the shallow core (LOM-1) representing the time interval of ~ 14.4 to 14.36 Ma. Globigerinoides trilobus, Globigerina bulloides, Cibicidoides spp. and Melonis pompilioides were chosen for analysis due to their variable ecological preferences within the water column and on the sea floor. We assessed problems concerning diagenetic alteration of foraminiferal calcite by using a single test approach when analysing δ18O and δ13C. Paleotemperature ranges turned out to be comparable with modern subtropical regions with no cooling trend as was expected at this period following the Middle Miocene Climatic Optimum. However, we find an interesting offset between the benthic species temperature response that is likely related to the different life-strategies of those species. Based on a combination of mentioned proxies it was possible to partly reconstruct δ18Oseawater for the studied locality and compare it with Mg/Ca derived temperature to obtain a possible correlation between temperature and salinity. We also investigated differences in surface and bottom water hydrography in comparison with the Mediterranean, and distinguish their different hydrographies. In addition, it was possible to identify dissimilar origins of surface and bottom water masses based on comparison of their isotopic composition.