C.J. Beets

Ocean circulation and iceberg discharge in the glacial North Atlantic: Inferences from unmixing of sediment size distributions

Variability in iceberg discharge and deep-ocean circulation in the North Atlantic during the last glacial period is inferred from the grain-size distribution and trace elemental composition of terrigenous sediments in a deep-sea core taken on Reykjanes Ridge, south of Iceland. End-member modeling of the grain-size distributions is used to unmix the signals of varying bottom-current speed and iceberg discharge. The size distribution within the silt fraction appears to be influenced by both factors. Based on Th-Sc-La relationship, we established that during the ice-rafted detritus events, continental material of likely Greenlandic origin increased to 87%, and that bottom-current–derived material contains to 40% mid-oceanic ridge fines, probably of Icelandic origin. Our results have important implications for the use of silt grain size as an indicator for paleocurrent speed in the glacial North Atlantic. We show that reconstructions of variations in bottom-current speed based on the raw grain-size data are opposite to inferences from the unmixed record. The latter indicates that deep-water convection decreased during periods of enhanced iceberg discharge, which is in general agreement with paleoceanographic reconstructions of the North Atlantic.

Pliocene/Pleistocene platform facies transition recorded in calciturbidites (Exuma Sound, Bahamas)

Abstract The composition of Pliocene-Pleistocene calciturbidites of ODP Hole 632A (Exuma Sound, Bahamas) has been determined by point-count analysis of thin-sections in order to correlate basin/platform events which influenced sedimentation. Strontium isotope chronostratigraphy has been used to correlate basin turbidite compositional variations to the evolution of the platform, as documented by magnetostratigraphy. Magnetostratigraphy of Great Bahama Bank cores shows that between 3.4 and 2.5 Ma (Late Pliocene) ooids and peloids became the dominant sediments across the platform. After this period the platform has been subjected to a series of subaerial exposure events. This led to slow accumulation on the platform and starvation of the basin. Around 1 Ma the platform was reflooded, the transgression being followed by an increased production of ooids. There is a change in calciturbidite composition in the earliest Pleistocene coinciding with a regional unconformity. Pleistocene turbidites, containing abundant skeletal material and ooids, overlie Pliocene/Early Pleistocene turbidites, rich in mud and skeletal material. The duration of the hiatus, represented by the unconformity, based on 87 Sr/ 86 Sr dating is 1.0-0.7 Ma. Prior to this hiatus sedimentation rates in the basin are an order of magnitude lower than present-day accumulation rates. We conclude that reflooding of the platform around 0.8 Ma led to shedding of both non-skeletal and skeletal sediments. The change of Great Bahama Bank from a Pliocene reef-rimmed skeletal platform to the present-day flat-topped bank, dominated by non-skeletal sediments, can be correlated with the observed change in turbidite composition.

Modeling the climate response to a massive methane release from gas hydrates

[1] The climate response to a massive release of methane from gas hydrates is simulated in two 2500-year-long numerical experiments performed with a three-dimensional, global coupled atmosphere-sea ice-ocean model of intermediate complexity. Two different equilibrium states were used as reference climates; the first state with preindustrial forcing conditions and the second state with a four times higher atmospheric CO2 concentration. These climates were perturbed by prescribing a methane emission scenario equivalent to that computed for the Paleocene/Eocene thermal maximum (PETM; similar to55.5 Ma), involving a sudden release of 1500 Gt of carbon into the atmosphere in 1000 years. In both cases, this produced rapid atmospheric warming (up to 10degreesC at high latitudes) and a reorganization of the global overturning ocean circulation. In the ocean, maximum warming (2-4degreesC) occurred at intermediate depths where methane hydrates are stored in the upper slope sediments, suggesting that further hydrate instability could result from the prescribed scenario.

A high resolution stable isotope record of the penultimate deglaciation in lake sediments below the city of Amsterdam, The Netherlands

Abstract A detailed record of the deglaciation history of the penultimate glacial to interglacial transition is given, mainly based on the stable isotopes of calcium carbonate-containing lake sediments in a glacially excavated depression below the city of Amsterdam. Initially, this depression, the Amsterdam Basin, formed part of a lake covering the western and central part of the Netherlands and extending for an unknown distance into the present North Sea. Annual layer counting shows that the lake drained after about a millennium, leaving shallow pools in the remaining depressions. The latter changed again into larger and deeper lakes including the Holland Lake during the rise of sea level in the early Eemian. Oxygen isotope values of the early lake sediments indicate an interglacial summer climate, but the nearness of dead-ice fields caused severe winters. The isotope record is furthermore characterized by the influx of large amounts of isotopically light water supplied by the river Rhine. A change to much colder conditions occurred simultaneously with the draining of the Holland Lake, as appears from oxygen isotope values and the sudden increase in non-arboreal pollen. This interval is correlated with the Kattegat Stadial and the sea-level standstill of Aladdins Cave on the Huon Peninsula of New Guinea. A short climate wiggle occurs at the end of this interval. The onset of the Eemian is marked by a rapid warming of ∼5°C which extends into local pollen-zone E3. Provided that our correlation of this cold interval with the sea-level standstill of Aladdins Cave is correct, the time interval between the earliest lake sediments in the Amsterdam Basin and the Eemian highstand took about 5–6 millennia.