Faunal analysis and Mg/Ca records of planktic foraminifera in sediment core SO189/2_039KL

Climate phenomena like the monsoon system, El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) are interconnected via various feedback mechanisms and control the climate of the Indian Ocean and its surrounding continents on various timescales. The eastern tropical Indian Ocean is a key area for the interplay of these phenomena and for reconstructing their past changes and forcing mechanisms. Here we present records of upper ocean thermal gradient, thermocline temperatures (TT) and relative abundances of planktic foraminifera in core SO 189-39KL taken off western Sumatra (0°47.400' S, 99°54.510' E) for the last 8 ka that we use as proxies for changes in upper ocean structure. The records suggest a deeper thermocline between 8 ka and ca 3 ka compared to the late Holocene. We find a shoaling of the thermocline after 3 ka, most likely indicating an increased occurrence of upwelling during the late Holocene compared to the mid-Holocene which might represent changes in the IOD-like mean state of the Indian Ocean with a more negative IOD-like mean state during the mid-Holocene and a more positive IOD-like mean state during the past 3 ka. This interpretation is supported by a transient Holocene climate model simulation in which an IOD-like mode is identified that involves an insolation-forced long-term trend of increasing anomalous surface easterlies over the equatorial eastern Indian Ocean.

Mg/Ca and Sr/Ca ratios of Hoeglundina elegans from surface sediments

Core top samples from Atlantic (Little Bahama Banks (LBB)) and Pacific (Hawaii and Indonesia) depth transects have been analyzed in order to assess the influence of bottom water temperature (BWT) and aragonite saturation levels on Mg/Ca and Sr/Ca ratios in the aragonitic benthic foraminifer Hoeglundina elegans. Both the Mg/Ca and Sr/Ca ratios in H. elegans tests show a general decrease with increasing water depth. Although at each site the decreasing trends are consistent with the in situ temperature profile, Mg/Ca and Sr/Ca ratios in LBB are substantially higher than in Indonesia and Hawaii at comparable water depths with a greater difference observed with increasing water depth. Because we find no significant difference between results obtained on "live" and "dead" specimens, we propose that these differences are due to primary effects on the metal uptake during test formation. Evaluation of the water column properties at each site suggests that in situ CO3 ion concentrations play an important role in determining the H. elegans Mg/Ca and Sr/Ca ratios. The CO3 ion effect is limited, however, only to aragonite saturation levels ([DeltaCO3]aragonite) below 15 µmol/kg. Above this level, temperature exerts a dominant effect. Accordingly, we propose that Mg/Ca and Sr/Ca in H. elegans tests can be used to reconstruct thermocline temperatures only in waters oversaturated with respect to the mineral aragonite using the following relationships: Mg/Ca = (0.034 ± 0.002)BWT + (0.96 ± 0.03) and Sr/Ca = (0.060 ± 0.002)BWT + (1.53 ± 0.03) (for [DeltaCO3]aragonite > 15 µmol/kg). The standard error associated with these equations is about ±1.1°C. Reconstruction of deeper water temperatures is complicated because in undersaturated waters, changes in Mg/Ca and Sr/Ca ratios reflect a combination of changes in [CO3] and BWT. Overall, we find that Sr/Ca, rather than Mg/Ca, in H. elegans may be a more accurate proxy for reconstructing paleotemperatures.

Dive depth and dive duration data of female southern elephant seals from Marion Island between 2004 and 2008 with links to datasets

The at-sea behaviour of marine top predators provides valuable insights into the distribution of prey species and strategies used by predators to exploit patchily distributed resources. We describe the water column usage and dive strategies of female southern elephant seals from Marion Island tracked between 2004 and 2008. Dives representing increases in forage effort were identified using a method that combines dive type analyses and the calculation of relative amounts of time that animals spend in the bottom phases of dives. Results from this analysis indicate that female elephant seals from Marion Island tend to display lower levels of forage effort closer to the island and display intensive opportunistic forage bouts that occur at a minimum distance of approximately 215 km from the island. Females from Marion Island dived deeper and for longer periods of time, compared to females from other populations. Most animals displayed positive diel vertical migration, evidently foraging pelagically on vertically migrating prey. A few animals displayed periods of reverse (negative) diel vertical migration, however, diving to deeper depths at night, compared to daytime. This behaviour is difficult to explain and prey species targeted during such periods unknown. Our results illustrate plasticity in foraging behaviour of southern elephant seals, as well as inter-population differences in forage strategies.

Pleistocene stable isotope record of planktonic foraminifera of ODP Site 138-847

We present Pleistocene oxygen and carbon isotope records from two planktonic foraminifer species (Globigerinoides sacculifer and Neogloboquadrina dutertrei) from Ocean Drilling Program Site 847 (0°16'N, 95°19'W; 3334 m water depth). An average sample resolution of 4500 yr was obtained by sampling at an interval of 15 cm through a continuous 35-m section from 0 to 1.15 Ma. Our d18O-based chronology is similar to that derived independently by astronomically tuning the gamma-ray attenuation porosity evaluator (GRAPE) record (Shackleton et al., 1995), though offsets as large as ± 30 k.y. occur on occasion. The surface waters at eastern equatorial Pacific Site 847, 380 km west of the Galapagos, are characterized by strong and constant upwelling, elevated nutrient concentrations, and high productivity. The isotopic composition of G. sacculifer (300-355 µm) reflects conditions in the thin-surface mixed layer, and the composition of N. dutertrei (355-425 µm) monitors the subsurface waters of the permanent shallow (10-40 m) thermocline. The Pleistocene d18O difference (N. dutertrei minus G. sacculifer, Dd18Od-s) averages 0.9 per mil and ranges from 0 per mil to 1.7 per mil. Neglecting species effects and shell size, the average Pleistocene d13C difference (G. sacculifer minus N. dutertrei, Dd13Cs-d) is 0.0 per mil and ranges from -0.5 per mil to 0.5 per mil. The Dd18Od-s and Dd13Cs-d records are used to infer vertical contrasts in upper ocean water temperature and nutrient concentration, though d13C may also be influenced by other factors, such as CO2 gas exchange. Variations in the isotopic differences are often synchronous with glacial/interglacial climate change. Glacial periods are characterized by smaller vertical contrasts in both temperature and nutrient concentration, and by notably greater accumulation rates of N. dutertrei and CaCO3. We attribute these responses to greater upwelling at the equatorial divergence. Superimposed on the glacial/interglacial Dd18Od-s pattern is a long-term trend possibly associated with the advection of Peru Current waters. The temporal fluctuations in the isotopic contrasts are strikingly similar to those observed at Site 851 (Ravelo and Shackleton, this volume), suggesting that the inferred changes in thermal and chemical profiles occurred over a broad region in the equatorial Pacific.