Eocene sedimentary calcium carbonate contents and stable isotope composition of benthic foraminifera

'Hyperthermals' are intervals of rapid, pronounced global warming known from six episodes within the Palaeocene and Eocene epochs (~65-34 million years (Myr) ago) (Zachos et al., 2005, doi:10.1126/science.1109004; 2008, doi:10.1038/nature06588; Roehl et al., 2007, doi:10.1029/2007GC001784; Thomas et al., 2000; Cramer et al., 2003, doi:10.1029/2003PA000909; Lourens et al., 2005, doi:10.1038/nature03814; Petrizzo, 2005, doi:10.2973/odp.proc.sr.198.102.2005; Sexton et al., 2006, doi:10.1029/2005PA001253; Westerhold et al., 2007, doi:10.1029/2006PA001322; Edgar et al., 2007, doi:10.1038/nature06053; Nicolo et al., 2007, doi:10.1130/G23648A.1; Quillévéré et al., 2008, doi:10.1016/j.epsl.2007.10.040; Stap et al., 2010, doi:10.1130/G30777.1). The most extreme hyperthermal was the 170 thousand year (kyr) interval (Roehl et al., 2007) of 5-7 °C global warming (Zachos et al., 2008) during the Palaeocene-Eocene Thermal Maximum (PETM, 56 Myr ago). The PETM is widely attributed to massive release of greenhouse gases from buried sedimentary carbon reservoirs (Zachos et al., 2005; 2008; Lourenbs et al., 2005; Nicolo et al., 2007; Dickens et al., 1995, doi:10.1029/95PA02087; Dickens, 2000; 2003, doi:10.1016/S0012-821X(03)00325-X; Panchuk et al., 2008, doi:10.1130/G24474A.1) and other, comparatively modest, hyperthermals have also been linked to the release of sedimentary carbon (Zachos et al., 2008, Lourens et al., 2005; Nicolo et al., 2007; Dickens, 2003; Panchuk et al., 2003). Here we show, using new 2.4-Myr-long Eocene deep ocean records, that the comparatively modest hyperthermals are much more numerous than previously documented, paced by the eccentricity of Earth's orbit and have shorter durations (~40 kyr) and more rapid recovery phases than the PETM. These findings point to the operation of fundamentally different forcing and feedback mechanisms than for the PETM, involving redistribution of carbon among Earth's readily exchangeable surface reservoirs rather than carbon exhumation from, and subsequent burial back into, the sedimentary reservoir. Specifically, we interpret our records to indicate repeated, large-scale releases of dissolved organic carbon (at least 1,600 gigatonnes) from the ocean by ventilation (strengthened oxidation) of the ocean interior. The rapid recovery of the carbon cycle following each Eocene hyperthermal strongly suggests that carbon was resequestered by the ocean, rather than the much slower process of silicate rock weathering proposed for the PETM (Zachos et al., 2005; 2003). Our findings suggest that these pronounced climate warming events were driven not by repeated releases of carbon from buried sedimentary sources (Zachos et al., 2008, Lourens et al., 2005; Nicolo et al., 2007; Dickens, 2003; Panchuk et al., 2003) but, rather, by patterns of surficial carbon redistribution familiar from younger intervals of Earth history.

Revised geomagnetic polarity timescale for the late Oligocene to early Miocene of ODP Site 177-1090

At Ocean Drilling Program (ODP) Site 1090 (subantarctic South Atlantic), benthic foraminiferal stable isotope data (from Cibicidoides and Oridorsalis) span the late Oligocene through early Miocene (~24-16 Ma) at a temporal resolution of ~5 ky. Over the same interval, a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity time scale (GPTS), thereby providing direct correlation of the isotope record to the GPTS. In an initial age model, we use the newly derived age of the Oligocene/Miocene (O/M) boundary of 23.0 Ma of Shackleton et al. (2000, doi:10.1130/0091-7613(2000)28<447:ACAFTO>2.0.CO;2), revised to the new astronomical calculation (La2003) of Laskar et al (2004, doi:10.1016/j.icarus.2004.04.005) to recalculate the spline ages of Cande and Kent (1995, doi:10.1029/94JB03098). We then tune the Site 1090 dekta18O record to obliquity using La2003. In this manner, we are able to refine the ages of polarity chrons C7n through C5Cn.1n. The new age model is consistent, within one obliquity cycle, with previously tuned ages for polarity chrons C7n through C6Bn from Shackleton et al. (2000) when rescaled to La2003. The results from Site 1090 provide independent evidence for the revised age of the Oligocene/Miocene boundary of 23.0 Ma. For early Miocene polarity chrons C6AAr through C5Cn, our obliquity-scale age model is the first to allow a direct calibration to the GPTS. The new ages are generally within one obliquity cycle of those obtained by rescaling the Cande and Kent (1995) interpolation using the new age of the O/M boundary (23.0 Ma) and the same middle Miocene control point (14.8 Ma) used by Cande and Kent (1995).

Reference datasets and pollen record from sediment core YAK, Yakumo, SW Hokkaido

This study reconstructs middle and late Holocene vegetation and climate dynamics in the Oshima Peninsula, SW Hokkaido, using the published method of biome reconstruction and modern analogue technique applied to the Yakumo pollen record (42°17'03''N, 140°15'34''E) spanning the last 5500 years. Two previously published matrices assigning Japanese plant/pollen taxa to the major vegetation types (biomes) are tested using a newly compiled dataset of 78 surface pollen spectra from Hokkaido. With both matrices showing strengths and weaknesses in reconstructing cool mixed and temperate deciduous forests of Hokkaido, the results suggest the necessity to consider the whole list of identified terrestrial pollen taxa for generating robust vegetation reconstructions for northern Japan. Applied to the fossil pollen data, both biome-reconstruction approaches demonstrate consistently that oak-dominated cool mixed forest spread in the study region between 5.5 and 3.6 cal ka BP and was subsequently replaced by beech-dominated temperate deciduous forest. The pollen-based climate reconstruction suggests this change in the vegetation composition was caused by a shift from cooler and drier than present climate to warmer and wetter, similar to modern conditions about 3.6 cal ka BP. Comparing the pollen-based reconstruction results with the published marine records from the NW Pacific, the reconstructed vegetation and climate dynamics can be satisfactorily explained by the greater role played by the warm Tsushima Current in the Sea of Japan and in the Tsugaru Strait during the middle and late Holocene. An increase in sea surface temperatures west and south of the study site would favour air temperature rise and moisture uptake and cause an increase in precipitation and snow accumulation in the western part of Hokkaido during the late Holocene.

Characteristics and radiocarbon ages of four retrogressive thaw slumps on Herschel Island and King Point

Four retrogressive thaw slumps (RTS) located on Herschel Island and the Yukon coast (King Point) in the western Canadian Arctic were investigated to compare the environmental, sedimentological and geochemical setting and characteristics of zones in active and stabilised slumps and at undisturbed sites. In general, the slope, sedimentology and biogeochemistry of stabilised and undisturbed zones differ, independent of their age or location. Organic carbon contents were lower in slumps than in the surrounding tundra, and the density and compaction of slump sediments were much greater. Radiocarbon dating showed that RTS were likely to have been active around 300 a BP and are undergoing a similar period of increased activity now. This cycle is thought to be controlled more by local geometry, cryostratigraphy and the rate of coastal erosion than by variation in summer temperatures.

Stable foraminifer isotope composition and age of ODP Sites 184-1147 and 184-1148, South China Sea

Sites 1147 (18°50.11'N, 116°33.28'E; water depth = 3246 m) and 1148 (18°50.17'N, 116°33.94'E; water depth = 3294 m) are located on the lowermost continental slope off southern China near the continent/ocean crust boundary of the South China Sea Basin. Site 1147 is located upslope ~0.45 nmi west of Site 1148. Three advanced piston corer holes at Site 1147 and two extended core barrel holes at Site 1148 were cored and combined into a composite (spliced) stratigraphic section, which provided a relatively continuous profile for the lower Oligocene to Holocene (Wang, Prell, Blum, et al., 2000, doi:10.2973/odp.proc.ir.184.2000; Jian, et al., 2001, doi:10.1007/BF02907088) for studying stratigraphy and paleoceanography. A total of 1047 planktonic foraminifers stable isotope measurements were performed on 975 samples covering the upper 409.58 meters composite depth (mcd) at ~42-cm intervals (Tables T1, T2), and a total of 1864 benthic foraminifers measurements were performed on 1650 samples in the upper 837.11 mcd at ~51-cm intervals (Tables T3, T4). We significantly improved the time resolution of the benthic stable isotope record in the upper 476.68 mcd by reducing the average sample spacing to ~29 cm. This translates into an average sampling resolution of ~16 k.y. for the Miocene sequence and ~8 k.y. for the Pliocene-Holocene interval, assuming a change in sedimentation rates from ~1.8 to ~3.5 cm/k.y., as suggested by shipboard stratigraphy. These data sets provide the basis for upcoming studies to establish an oxygen isotope stratigraphy and examine the Neogene evolution of deep and surface water signatures (temperature, salinity, and nutrients) in the South China Sea.

Coral oxygen isotope and Sr/Ca data from the Northern Gulf of Aqaba (Red Sea)

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.

Bacterial activity in sediments of the deep Arabian Sea

In the Arabian Sea, productivity in the surface waters and particle flux to the deep sea are controlled by monsoonal winds. The flux maxima during the South-West (June-September) and the North-East Monsoon (December-March) are some of the highest particle fluxes recorded with deep-sea sediment traps in the open ocean. Benthic microbial biomass and activities in surface sediments were measured for the first time in March 1995 subsequent to the NE-monsoon and in October 1995 subsequent to the SW-monsoon. These measurements were repeated in April/May 1997 and February/March 1998, at a total of six stations from 1920 to 4420 m water depth. This paper presents a summary on the regional and temporal variability of microbial biomass, production, enzyme activity, degradation of 14C-labeled Synechococcus material as well as sulfate reduction in the northern, western, eastern, central and southern Arabian deep sea. We found a substantial regional variation in microbial biomass and activity, with highest values in the western Arabian Sea (station WAST), decreasing approximately threefold to the south (station SAST). Benthic microbial biomass and activity during the NE-monsoon was as high or higher than subsequent to the SW-monsoon, indicating a very rapid turnover of POC in the surface sediments. This variation in the biomass and activity of the microbial assemblages in the Arabian deep sea can largely be explained by the regional and temporal variation in POC flux. Compared to other abyssal regions, the substantially higher benthic microbial biomasses and activities in the Arabian Sea reflect the extremely high productivity of this tropical basin.