ALBERT

Description: The long-chain diol index (LDI) is a new organic sea surface temperature (SST) proxy based on the distribution of long-chain diols. It has been applied in several environments but not yet in subpolar regions. Here, we tested the LDI on surface sediments and a sediment core from the Sea of Okhotsk, which is the southernmost seasonal sea ice covered region in the Northern Hemisphere, and compared it with other organic temperature proxies, i.e. Uk'37 and TEXL86. In the surface sediments, the LDI is correlated with autumn sea surface temperature, similar to the Uk'37 but different from the TEXL86 that correlates best with summer sea subsurface temperature. Remarkably, the obtained local LDI calibration was significantly different from the global core-top calibration. We used the local LDI calibration to reconstruct past SST changes in the central Sea of Okhotsk. The LDI-SST record shows low glacial (Marine Isotope Stage, MIS 2, 4, 6) and high interglacial (MIS 1 and MIS 5) temperatures and follows the same pattern as the Uk'37-SST and a previously published TEXL86 temperature record. Similar to the modern situation, the reconstructed temperatures during the interglacials likely reflect different seasons, i.e. summer for the TEXL86 and autumn for Uk'37 and LDI. During glacials, the reconstructed temperatures of all three proxies are similar to each other, likely reflecting summer temperatures as this was the only season free of sea ice. Our results suggest that the LDI is a suitable proxy to reconstruct subpolar sea water temperatures.

Description: The diatom valves were counted at magnifications of 1000 times; at least 400 valves were counted per sample. Percentages are based on the total diatom sum. The samples with poor diatom preservation (〈100 diatom valves) were only used for estimation of the diatom abundance, while were not used for the discussions of diatom percentages.

Description: The abundance of iron and manganese were detected by using non-destructive X-ray Fluorescence (XRF) scanning technique. Continuous downcore measurements of element variations were performed in the ITRAX XRF Core Scanner Laboratory, Department of Geosciences, National Taiwan University. All U-channels were scanned by using the 3 kW Mo source and were analyzed at 30 kV/24 mA, 2 mm resolution with a exposure time of 30 s. The count value of each element was calculated from element peak areas of original XRF spectra by the Q-Spec software provided by COX Analytical Systems.

Description: Magnetic measurements of U-channel samples were measured by the Bartington MS2 magnetic susceptibility system with an ASC auto-tracking rail and a cryogenic magnetometer (2G 755 SRM) in the shielding room of the paleomagnetic laboratory at the Institute of Earth Sciences, Academia Sinica, Taiwan. The Anhysteretic Remanent Magnetization (ARM) was applied with a DC bias field of 1 Gauss under a 100 mT alternating field. All U-channel measurements are within 1 cm interval.

Description: We provide an integrated stratigraphy for the southern margin of the Indo-Pacific Warm Pool back to 3.2 million years ago (Ma) from ODP Site 1115, Hole B (9o11'S, 151o34'E, water depth 1149 m, 0-217 m below sea floor), the only hole with core recovery beyond 4.4 m. An initial age model based on 14 calcareous nannofossil datums, 5 planktonic foraminiferal biostratigraphic events and 12 magnetic polarity reversals is refined by planktonic foraminiferal Globigerinoides sacculifer (300-355 μm) δ18O stratigraphy. The planktonic δ18O record with an average 3-4 kyr resolution matches the global stack LR04 (Lisiecki and Raymo, 2005). The 41-kyr obliquity and 100-kyr eccentricity patterns show good correlation between the LR04 and the dataset presented herein. The precession amplitude matches eccentricity, supporting the established stratigraphy. Sedimentation rates at Hole 1115B have decreased from 9.3 to 4.5 cm/kyr during the past 3.2 Ma toward present, mainly due to the gradual deepening of the Woodlark Basin. The planktonic δ18O record from Site 1115 matches the global climate evolution well.

Description: Recent reduction in high-latitude sea ice extent demonstrates that sea ice is highly sensitive to external and internal radiative forcings. In order to better understand sea ice system responses to external orbital forcing and internal oscillations on orbital timescales, here we reconstruct changes in sea ice extent and summer sea surface temperature (SSST) over the past 130,000 yrs in the central Okhotsk Sea. We applied novel organic geochemical proxies of sea ice (IP25), SSST (TEXL86) and open water marine productivity (a tri-unsaturated highly branched isoprenoid and biogenic opal) to marine sediment core MD01-2414 (53°11.77'N, 149°34.80'E, water depth 1123 m). To complement the proxy data, we also carried out transient Earth system model simulations and sensitivity tests to identify contributions of different climatic forcing factors. Our results show that the central Okhotsk Sea was ice-free during Marine Isotope Stage (MIS) 5e and the early-mid Holocene, but experienced variable sea ice cover during MIS 2-4, consistent with intervals of relatively high and low SSST, respectively. Our data also show that the sea ice extent was governed by precession-dominated insolation changes during intervals of atmospheric CO2 concentrations ranging from 190 to 260 ppm. However, the proxy record and the model simulation data show that the central Okhotsk Sea was near ice-free regardless of insolation forcing throughout the penultimate interglacial, and during the Holocene, when atmospheric CO2 was above ~260 ppm. Past sea ice conditions in the central Okhotsk Sea were therefore strongly modulated by both orbital-driven insolation and CO2-induced radiative forcing during the past glacial/interglacial cycle.