Stable oxygen and carbon isotopes from ODP sites on Maud Rise and Kerguelen Plateau

Paleogene stable oxygen and carbon isotopes were measured in formainifera from ODP Sites 689 and 690 at Maud Rise in the Atlantic Ocean sector of the Southern Ocean, and from Sites 738, 744, 748 and 749 at the southern Kerguelen Plateau in the Indian Ocean sector. These data were compared with sedimentological data from the same sample set. Both benthic and planktic d18O values document a cooling trend beginning around 49.5 Ma at all sites. During the late middle Eocene planktic d18O values indicate a steepening latitudinal temperature gradient from 14°C at the northern sites towards 10°C at the southernmost sites. Terrigeneous sand grains of probably ice rafted origin and clay mineral assemblages point to the existence of a limited East Antarctic ice cap with some glaciers reaching sea level as early as middle Eocene time around 45.5 Ma. Between 45 and 40 Ma, average paleotemperatures were between 5° and 7°C in deep and intermediate water masses, while near-surface water masses ranged between 6° and 10°C. During the late Eocene, between 40 and 36 Ma, average temperatures further decreased to 4°-5°C in the deep and intermediate water masses and to 5°-8°C near the sea surface. Abruptly increasing d18O values at approximately 35.9 Ma exactly correlate with a sharp pulse in the deposition of ice-rafted material on the Kerguelen Plateau, a dramatic change in clay mineral composition, and an altered Southern Ocean circulation indicated by a differentiation of benthic d13C values between sites, increasing opal concentrations and decreasing carbonate contents. For planktic and benthic foraminifera this d18O increase ranges between 1.0 and 1.3 per mil, and between 0.9 and 1.4 per mil, respectively. We favour a hypothesis that explains most of the d18O shift at 35.9 Ma with a buildup of a continental East Antarctic ice sheet. Consequently, relatively warm Oligocene Antarctic surface water temperatures probably are explained by a temperate, wet-based nature of the ice sheet. This would also aid in the fast build-up of an ice sheet by enhancing the moisture transport on to the continent.

Turbulence measurements performed in the North Atlantic during the James Cook cruise JC087, March 2013

During the cruise the turbulence probe MSS050 was used for microstructure measurements. The profiler is produced by Sea and Sun Technology GmbH in co-operation with ISW Wassermesstechnik. The probe was equipped with 2 velocity microstructure shear sensors, a microstructure temperature sensor, standard CTD sensors for precision measurements, a vibration control sensor, a two component tilt sensor, a fluorescence sensor and surface detection sensor (SD) to indicate the water surface hit at rising measurements. The sampling rate for all sensors is 1024 samples per second, the resolution 16 bit. During the MSS measurements, the ship was moving with speed approx. 0.5-1.0 knots with respect to the water against the wind. In order to take into account the intermittent nature of marine turbulence, repeated MSS measurements were carried out in bursts of typically 10 profiles per station. The measurement interval was approximately 7 (10) min for a profile to 140 (200) dbar. During JC87 cruise 8 series of turbulence measurements were conducted. Fluorescence data was obtained on the 5 of them.