Clay mineraology and grain size composition of surface sediments from South Pacific Ocean

The reconstruction of low-latitude ocean-atmosphere interactions is one of the major issues of (paleo-)environmental studies. The trade winds, extending over 20° to 30° of latitude in both hemispheres, between the subtropical highs and the intertropical convergence zone, are major components of the atmospheric circulation and little is known about their long-term variability on geological time-scales, in particular in the Pacific sector. We present the modern spatial pattern of eolian-derived marine sediments in the eastern equatorial and subtropical Pacific (10°N to 25°S) as a reference data set for the interpretation of SE Pacific paleo-dust records. The terrigenous silt and clay fractions of 75 surface sediment samples have been investigated for their grain-size distribution and clay-mineral compositions, respectively, to identify their provenances and transport agents. Dust delivered to the southeast Pacific from the semi- to hyper-arid areas of Peru and Chile is rather fine-grained (4-8 µm) due to low-level transport within the southeast trade winds. Nevertheless, wind is the dominant transport agent and eolian material is the dominant terrigenous component west of the Peru-Chile Trench south of ~ 5°S. Grain-size distributions alone are insufficient to identify the eolian signal in marine sediments due to authigenic particle formation on the sub-oceanic ridges and abundant volcanic glass around the Galapagos Islands. Together with the clay-mineral compositions of the clay fraction, we have identified the dust lobe extending from the coasts of Peru and Chile onto Galapagos Rise as well as across the equator into the doldrums. Illite is a very useful parameter to identify source areas of dust in this smectite-dominated study area.

Lead isotopes of feldspar in North Atlantic Ocean sediments

The provenance of ice-rafted debris (IRD) deposited in the North Atlantic before, during, and after Heinrich event 2 has been determined through measuring the lead isotopic composition of single feldspar grains and multiple-grain composites from the larger than 150-µm size fraction, from cores from the eastern and western North Atlantic and from the Labrador Sea. Single-grain analyses are used to identify the specific continental sources of the IRD, whereas composite samples are used to assess the relative IRD contributions from different sources. All single grains from Heinrich layer 2 (H 2) as well as H 2 composites plot along a correlation line on a 207Pb/204Pb versus 206Pb/204Pb diagram characteristic of the Churchill province of the Canadian shield. This is yet another strong piece of evidence that this Heinrich event was dominated by a massive iceberg discharge of the Laurentide ice sheet lobe located over Hudson Bay. In contrast, single grains from the ambient glacial sediment (above and below H 2) have multiple sources: many of them also lie along the correlation line with H 2 grains, but many others have Pb signatures consistent with derivation from the Grenville province and the Appalachian range in North America and possibly from Scandinavia and Greenland. Composites from the ambient sediment generally lie well to the right of the H 2 reference line in agreement with the results of the single-grain analyses. The evidence provided by lead isotopes regarding the dominant role played by the Hudson Bay lobe of the Laurentide ice sheet in the development of the Heinrich events lends support to the binge/purge model advanced by MacAyeal [1993a, b] that invokes trapping of geothermal heat by the base of the icecap and subsequent basal melting as the mechanism that triggered the Heinrich events.

Tab. 1: Rates of advance of the frontal push-moraines of 19 lobes in front of an approx. 4.5 km long margin of the Kötlujökull

In the last years masses of ice, about 5 km long, have been protruding from the lowest part of an advancing glacier margin of the Kötlujökull in Southern Iceland. In the summer of 1983, they appeared as sediment-covered lobes, 10-60 m long, bordering the glacier rnargin like agarland. 1 to 3 push-rnoraines without ice core, rnostly sickle-shaped, occured first in the frontal parts of the lobes: behind thern came several ice-cored moraines with heights of up to several metres. The active ice in front of the precipice of the glacier is called the "glacier-foot" in this paper. The digging out of 9 lobes and the measuring of the advance of 19 lobes showed that in most cases this glacierfoot had split up at its distal end into several plate- or stem-shaped pieces of ice which were situated one upon the other, separated by moraine deposits and proceeding irregularly into the foreland at the rate of several mm/h, The sometimes different rate of advance in the same lobe and different rates of advanee in adjoining lobes (some being entirely inactive) point to a type of rnovement which is independent of the general advance of the glacier. Research in the winter of 1983/84 showed less activity in 3 examined lobes, but the activity had not ceased. The advancement of the lower parts of the glacier-foot into and across the sands of the foreland implies the following genesis of pushmoraines: Shoving off a plate of sand, folding it and pushing it over the foreland at average rates of up to 7,2 mm/h, according to the investigations in thc summer of 1983. At a certain stage of the folding process, new folds begin to develop in front of the old, and the old folds are shifted onto the backslope of thc folds in front of them until they are completely unired. In this way, "püe-moraines" arise, which become higher and higher. They include two or more folds declining towards the glacier. Systems of small moraines presumably of the same genesis occur on old moraine areas in front of the Kötlujökull. The possible cause of formation of a glacier-foot is discussed, and the moraines of the Kötlujökull are compared with certain pleistocene push-moraines.

Sedimentology and age on profile Lz1023

A 270 cm long sediment sequence was recovered with a piston corer from east lobe Bonney, Taylor Valley, Antarctica, and characterized according to its sedimentological, mineralogical, and geochemical properties. It is the first record of such length recovered from east lobe Bonney. The sediment core is mainly composed of halite crystals of different sizes, water, and a relatively low and stable proportion of clastic particles. Although the sediment surface was probably disturbed by the coring process and absence or low contents of organic material or carbonates hampers the establishment of a robust chronology by radiocarbon dating, the core probably contains at least several hundred years of information about the history of the lake and the Bonney basin. Variations in halite crystal sizes and amount as well as variations in the composition of clastic material can be related to past lake level changes and evaporation cycles.