Ice rafted debris distribution in sediment core CRP-1

Lonestone abundances in CRP-1 were investigated using three methods: core examination at Cape Roberts Camp, analysis of digital core images and follow-up core examination. For all images of split-core, we determined size and depth of every detectable lonestone larger than 3 mm. Lonestone abundance decreases exponentially with clast size. Although no significant depth-dependent variations in lonestone size distribution were detected, a strong 0.5-0.7 m abundance periodicity, of unknown origin, is evident within diamicts. Lonestone volume percentage was estimated from size distribution: most size classes contribute approximately the same volume to the total. Sizes >16 mm have rare enough lonestones that their counts are nonrepresentative when based on short intervals of split core. This problem does not affect total counts significantly, but the volume analysis needs to be confined to <= 6 mm lonestones to avoid instability induced by rare and nonrepresentative larger lonestones. If lonestone abundance can be used as an indicator of glacial proximity, then our CRP-1 lonestone abundance logs confirm the overall character of previously inferred variations in relative distance to the ice margin. Large-scale changes in lonestone abundance also reflect the CRP-1 sequence stratigraphy, with individual sequences generally characterised by basal lonestone-rich diamict overlain by lonestone-poor sands and muds. The relationship between glacial proximity and lonestone abundance within diamicts and within sand-mud intervals is, however, less certain. For example, two or three gradual lonestone increases may indicate regressions during glacial advances, in contrast to the more common CRP-l pattern of dominantly transgressive sequences.

Total organic carbon content and redox-sensitiv trace metal concentrations of selected late Cenoman claystones from DSDP Hole 93-603B

The Cenomanian/Turonian (C/T) intervals at DSDP Sites 105 and 603B from the northern part of the proto-North Atlantic show high amplitude, short-term cyclic variations in total organic carbon (TOC) content. The more pronounced changes in TOC are also reflected by changes in lithology from green claystones (TOC<1%) to black claystones (TOC>1%). Although their depositional history was different, the individual TOC cycles at Sites 105 and 603B can be correlated using stable carbon isotope stratigraphy. Sedimentation rates obtained from the isotope stratigraphy and spectral analyses indicate that these cycles were predominately precession controlled. The coinciding variations in HI, OI, delta13Corg and the abundance of marine relative to terrestrial biomarkers, as well as the low abundance of lignin pyrolysis products generated from the kerogen of the black claystones, indicate that these cyclic variations reflect changes in the contribution of marine organic matter (OM). The cooccurrence of lamination, enrichment of redox-sensitive trace metals and presence of molecular fossils of pigments from green sulfur bacteria indicate that the northern proto-North Atlantic Ocean water column was periodically euxinic from the bottom to at least the base of the photic zone (<150 m) during the deposition of the black claystones. In contrast, the green claystones are bioturbated, are enriched in Mn, do not show enrichments in redox-sensitive trace metals and show biomarker distributions indicative of long oxygen exposure times, indicating more oxic water conditions. At the same time, there is evidence (e.g., abundance of biogenic silica and significant 13C-enrichment for OC of phytoplanktic origin) for enhanced primary productivity during the deposition of the black claystones. We propose that increased primary productivity periodically overwhelmed the oxic OM remineralisation potential of the bottom waters resulting in the deposition of OM-rich black claystones. Because the amount of oxygen used for OM remineralisation exceeded the amount supplied by diffusion and deep-water circulation, the northern proto-North Atlantic became euxinic during these periods. Both Sites 105 and 603B show trends of continually increasing TOC contents and HI values of the black claystones up section, which most likely resulted from both enhanced preservation due to increased anoxia and increased production of marine OM during oceanic anoxic event 2 (OAE2).

Petrophysical investigations on sediment core CRP-2A from the Ross Sea, Antarctica

A suite of petropysical measurements - velocity versus pressure, bulk density, porosity, matrix density, and magnetic susceptibility -was undertaken on 63 core plugs from CRP-2A. These data are used to calibrate neutron, resistivity, and magnetic susceptibility well logs. Agreement between core-plug magnetic susceptibility measurements and both well-log and whole-core data is excellent. Comparison of core-plug bulk densities with continious well-log density records shows very good agreement. Core-plug measurements of matrix density permit conversion of the well-log and whole-core density records to porosity. Sands and muds exhibit similar downhole compaction patterns, and both patterns are consistent with 250 ± 150 m of exhumation. Pervasive cementation, particularly in the lower half of the core, has affected many CRP-2A petrophysical parameters: (1) fractional porosities are reduced by about 0.05 - 0.10 in the lower part of the hole; (2) velocity and porosity rebound are much smaller than is usually observed for unconsolidated sediments with burial depths similar to CRP-2A; (3) velocities are unusually insensitive to pressure, suggesting that any exhumation-induced microcracks have been scaled subsequently; (4) the velocity/porosity relationship lacks the characteristic signature of exhumation-induced microcracks; (5) the velocity/porosity relationship changes with depth, indicating downhole increase in consolidation; (6) Vp/Vs ratios of the highest-porosity sediments are unusually low, implying enhancement of framework stiffness.

Stable isotopes and XRF data over the Paleocene-Eocene Thermal Maximum from IODP Sites U1403 and U1409

During the Paleocene-Eocene Thermal Maximum (PETM) about 56 million years ago, thousands of petagrams of carbon were released into the atmosphere and ocean in just a few thousand years, followed by a gradual sequestration over approximately 200,000 years. If silicate weathering is one of the key negative feedbacks that removed this carbon, a period of seawater calcium carbonate saturation greater than pre-event levels is expected during the event's recovery phase. In marine sediments, this should be recorded as a temporary deepening of the depth below which no calcite is preserved - the calcite compensation depth (CCD). Previous and new sedimentary records from sites that were above the pre-PETM calcite compensation depth show enhanced carbonate accumulation following the PETM. A new record from an abyssal site in the North Atlantic that lay below the pre-PETM calcite compensation depth shows a period of carbonate preservation beginning about 70,000 years after the onset of the PETM, providing the first direct evidence for an over-deepening of the calcite compensation depth. This record confirms an overshoot in ocean carbonate saturation during the PETM recovery. Simulations with two earth system models support scenarios for the PETM that involve both a large initial carbon release followed by prolonged low-level emissions, consistent with the timing of CCD deepening in our record. Our findings indicate that sequestration of these carbon emissions was most likely the result of both globally enhanced calcite burial above the calcite compensation depth and, at least in the North Atlantic, by a temporary over-deepening of the calcite compensation depth.

(Appendix) Grain-size distribution, calcium carbonate content and proportion of cemented aggregates in ODP Site 133-820

The textural and compositional characteristics of the 400 m sequence of Pleistocene wackestones and packstones intersected at Ocean Drilling Program (ODP) Site 820 reflect deposition controlled by fluctuations in sea-level, and by variations in the rate of sediment supply. The development of an effective reefal barrier adjacent to Site 820, between 760 k.y. and 1.01 Ma, resulted in a marked reduction in sediment accumulation rates on the central Great Barrier Reef outermost shelf and upper slope. This marked change corresponds with the transition from sigmoidal prograding seismic geometry in the lower 254 m of the sequence, to aggradational geometry in the top 146 m. The reduction in the rate of sediment accumulation that followed development of the reefal barrier also caused a fundamental change in the way in which fluctuations in sea-level controlled sediment deposition. In the lower, progradational portion of the sequence, sea-level cyclicity is represented by superimposed coarsening-upward cycles. Although moderately calcareous throughout (mostly 35%-75% CaCO3), the depositional system acted in a similar manner to siliciclastic shelf depositional systems. Relative sea-level rises resulted in deposition of more condensed, less calcareous, fine, muddy wackestones at the base of each cycle. Sea-level highstands resulted in increased sedimentation rates and greater influx of coarse bioclastic material. Continued high rates of sedimentation of both coarse bioclastic material and mixed carbonate and terrigenous mud marked falling and low sea-levels. This lower part of the sequence therefore is dominated by coarse packstones, with only thin wackestone intervals representing transgressions. In contrast, sea-level fluctuations following formation of an effective reefal barrier produced a markedly different sedimentary record. The more slowly deposited aggradational sequence is characterized by discrete thin interbeds of relatively coarse packstone within a predominantly fine wackestone sequence. These thin packstone beds resulted from relatively low sedimentation rates during falling and low sea-levels, with much higher rates of muddy sediment accumulation during rising and high sea-levels. The transition from progradational to aggradational sequence geometry therefore corresponds to a transition from a "siliciclastic-type" to a "carbonate-type" depositional system.