Sediment composition in NE Greenland Fjords, Shelf, and Slope

In order to document changes in Holocene glacier extent and activity in NE Greenland (~73° N) we study marine sediment records that extend from the fjords (PS2631 and PS2640), across the shelf (PS2623 and PS2641), to the Greenland Sea (JM07-174GC). The primary bedrock geology of the source areas is the Caledonian sediment outcrop, including Devonian red beds, plus early Neoproterozoic gneisses and early Tertiary volcanics. We examine the variations in colour (CIE*), grain size, and bulk mineralogy (from X-ray diffraction of the <2 mm sediment fraction). Fjord core PS2640 in Sofia Sund, with a marked red hue, is distinct in grain size, colour and mineralogy from the other fjord and shelf cores. Five distinct grain-size modes are distinguished of which only one is associated with a coarse ice-rafting signal - this mode is rare in the mid- and late Holocene. A sediment unmixing program (SedUnMixMC) is used to characterize down-core changes in sediment composition based on the upper late Holocene sediments from cores PS2640 (Sofia Sund), PS2631 (Kaiser Franz Joseph Fjord) and PS2623 (south of Shannon Is), and surface samples from the Kara Sea (as an indicator of transport from the Russian Arctic shelves). Major changes in mineral composition are noted in all cores with possible coeval shifts centred c. 2.5, 4.5 and 7.5 cal. ka BP (±0.5 ka) but are rarely linked with changes in the grain-size spectra. Coarse IRD (>2 mm) and IRD-grain-size spectra are rare in the last 9-10 cal. ka BP and, in contrast with areas farther south (~68° N), there is no distinct IRD signal at the onset of neoglaciation. Our paper demonstrates the importance of the quantitative analysis of sediment properties in clarifying source to sink changes in glacial marine environments.

Toba Tephra across India

A controversy currently exists regarding the number of Toba eruptive events represented in the tephra occurrences across peninsular India. Some claim the presence of a single bed, the 75,000-yr-old Toba tephra; others argue that dating and archaeological evidence suggest the presence of earlier Toba tephra. Resolution of this issue was sought through detailed geochemical analyses of a comprehensive suite of samples, allowing comparison of the Indian samples to those from the Toba caldera in northern Sumatra, Malaysia, and, importantly, the sedimentary core at ODP Site 758 in the Indian Ocean - a core that contains several of the earlier Toba tephra beds. In addition, two samples of Toba tephra from western India were dated by the fission-track method. The results unequivocally demonstrate that all the presently known Toba tephra occurrences in peninsular India belong to the 75,000 yr B.P. Toba eruption. Hence, this tephra bed can be used as an effective tool in the correlation and dating of late Quaternary sedimentary sequences across India and it can no longer be used in support of a middle Pleistocene age for associated Acheulian artifacts.

Diatoms and laminae of Mediterranean sapropels

The origins of sapropels (sedimentary layers rich in organic carbon) are unclear, yet they may be a key to understanding the influence of climate on ocean eutrophication, the mechanisms of sustaining biological production in stratified waters and the genesis of petroleum source rocks (Rohling, 1994, doi:10.1016/0025-3227(94)90202-X; Castradori, 1993, doi:10.1029/93PA00756; Calvert et al., 1992, doi:10.1038/359223a0). Recent microfossil studies of foraminifera (Rohling, 1994, doi:10.1016/0025-3227(94)90202-X) and calcareous nannofossils (Castradori, 1993, doi:10.1029/93PA00756) have focused attention on a deep chlorophyll maximum as a locus for the high production inferred (Calvert et al., 1992, doi:10.1038/359223a0) for sapropel formation, but have not identified the agent responsible. Here we report the results of a high-resolution, electron-microscope-based study of a late Quaternary laminated sapropel in which the annual flux cycle has been preserved. We find that much of the production was by diatoms, both mat-forming and other colonial forms, adapted to exploit a deep nutrient supply trapped below surface waters in a stratified water column. Reconstructed organic-carbon and opal fluxes to the sediments are comparable to those at high-productivity sites in today's oceans, and calculations based on diatom Si/C ratios suggest that the high organic-carbon content of sapropels may be entirely accounted for by sedimenting diatoms. We propose that this style of production may have been common in ancient Palaeogene and Cretaceous seas, environments for which conventional appeals to upwelling-driven production to account for the occurrence of diatomites, and some organic-carbon-rich sediments, have never seemed wholly appropriate.

Age determination of sediment cores from the Panama Basin

High-resolution percent Corg and delta18Oforam records obtained from Panama Basin core Atlantis II 54-25PC and additional data from nearby core P7 show that enhanced burial of organic carbon has characterized every major glacial period for the last 500 kyr in that area. Both Corg concentration and mass accumulation rate profiles exhibit a sawtooth pattern with maxima occurring typically in the later stages of glacial periods. Comparison with dust records suggests that the carbon accumulation rate profile reflects both the upwelling history and a variable rate of iron input during the late Quaternary. The sawtooth character may derive from increased wind velocities and rates of upwelling during glacials which are indirectly related to ice volume (Sarnthein et al., 1988). The rapid decline in export production at the end of glacials in the equatorial Pacific may be attributed to the retreat of ice sheets (thus reduced wind velocities and upwelling) coupled with a coincident decline in atmospheric dust load and/or delivery rate. The Corg accumulation rate profiles do not correlate well with atmospheric CO2 records. For example, atmospheric CO2 was already at a minimum 40 kyr ago when production in the Panama Basin began increasing dramatically, commensurate with an increase in global dust levels. Using the relationship between the degree of photosynthetic fractionation and the concentration of free CO2 in the surface ocean postulated by Popp et al. (1989), delta13Corg measurements made on core P7 show that Panama Basin surface waters have been supplying CO2 to the atmosphere continually for at least the last 50 kyr. There is no evidence for a flux of CO2 into the surface ocean in this area at any time during this period despite the higher production. If the Panama Basin cores are representative of the eastern and central equatorial Pacific, then these observations weaken the influence on CO2 drawdown postulated for increased glacial productivity at low latitudes.

(Table 3) Age-related trends of desert pavement properties in the Transantarctic Mountains

Compared to mid-latitude deserts, the properties, formation and evolution of desert pavements and the underlying vesicular layer in Antarctica are poorly understood. This study examines the desert pavements and the vesicular layer from seven soil chronosequences in the Transantarctic Mountains that have developed on two contrasting parent materials: sandstone-dolerite and granite-gneiss. The pavement density commonly ranges from 63 to 92% with a median value of 80% and does not vary significantly with time of exposure or parent material composition. The dominant size range of clasts decreases with time of exposure, ranging from 16-64 mm on Holocene and late Quaternary surfaces to 8-16 mm on surfaces of middle Quaternary and older age. The proportion of clasts with ventifaction increases progressively through time from 20% on drifts of Holocene and late Quaternary age to 35% on Miocene-aged drifts. Desert varnish forms rapidly, especially on dolerite clasts, with nearly 100% cover on surfaces of early Quaternary and older age. Macropitting occurs only on clasts that have been exposed since the Miocene. A pavement development index, based on predominant clast-size class, pavement density, and the proportion of clasts with ventifaction, varnish, and pits, readily differentiated pavements according to relative age. From these findings we judge that desert pavements initially form from a surficial concentration of boulders during till deposition followed by a short period of deflation and a longer period of progressive chemical and physical weathering of surface clasts. The vesicular layer that underlies the desert pavement averages 4 cm in thickness and is enriched in silt, which is contributed primarily by weathering rather than eolian deposition. A comparison is made between desert pavement properties in mid-latitude deserts and Antarctic deserts.

Calcareous nannofossils of the Soithern Coral Sea

Leg 90 of the Deep Sea Drilling Project drilled 18 holes at eight sites (Sites 587-594) on several shallow-water platforms in the southern Coral Sea, Tasman Sea, and southwestern Pacific Ocean. The results from an additional hole (Hole 586B) drilled at Site 586 during Leg 89 are included in this report. Together, these sites form a latitudinal traverse which extends from the equator (Site 586) to 45°S (Site 594) and includes all the major water masses from tropical to subantarctic. Samples recovered at these sites range in age from middle Eocene to late Quaternary. The calcareous nannoplankton biostratigraphy for Leg 90 has divided into two parts: part 1, the Neogene and Quaternary of Sites 586-594. (this chapter); and part 2, the Paleogene of Sites 588, 592, and 593 (Martini, 1986). A slightly modified version of the Martini (1971) standard Tertiary and Quaternary zonation scheme was used to make age determinations on over 700 samples. All of the relevant Neogene and Quaternary zone-defining nannoplankton are present at Sites 586-591 (0°-30°S) but become increasingly rare or are absent at Sites 592-594 (35°-45°S). Species diversity increases southward from the equator (Site 586) and reaches a peak at 20°S (Site 587). A decrease at 25°S (Site 588) and 30°S (Sites 589-591) is followed by an increase in species diversity at 35°S (Site 592). South of 35°S, species diversity again decreases and reaches a low at 45 °S (Site 594). Species diversity for all sites as a group generally increases through the early, middle, and late Miocene, reaches a peak in the early Pliocene, then gradually decreases through the late Pliocene and Quaternary

CaCO3 size distribution in surface sediments

Lysocline reconstructions play an important role in scenarios purporting to explain the lowered atmospheric CO2 content of glacial time. These reconstructions are based on indicators such as the CaCO3 content, the percent of coarse fraction, the ratio of fragments to whole foraminifera shells, the ratio of solution-susceptible to solution-resistant species, and the ratio of coarse to fine CaCO3. All assume that changes with time in the composition of the input material do not bias the result. However, as the composition of the input material does depend on climate, none of these indicators provides an absolute measure of the extent of dissolution. In this paper we evaluate the reliability of the ratio of >63 µm CaCO3 to total CaCO3 as a dissolution indicator. We present here results that suggest that in today's tropics this ratio appears to be determined solely by CO3= ion concentration and water depth (i.e., the saturation state of bottom waters). This finding offers the possibility that the size fraction index can be used to reconstruct CO3= ion concentrations for the late Quaternary ocean to an accuracy of ±5 µmol/kg.

Chemical composition of phosphorite nodules off Peru

Phosphorite nodules collected during Cruise 8 of R/V Dmitry Mendeleev off the Peruvian coast are represented by three lithologic types: friable, compacted, and solid. As the nodules are progressively compacted, contents of phosphate components increase and of non-phosphate components decrease. Isotope composition of uranium in the friable nodules is the same as in ocean water, and diatoms contained in the nodules are represented by recent species indicating Late Quaternary age of the nodules.