162 Preliminary Report



Clearly one of the most exciting results of this leg is the mere fact that so many continuous sequences with high sedimentation rates were recovered from areas where major components of the climate system can be monitored (Fig. 7a)(Fig. 7b). In addition, a complete magnetostratigraphic record from the interval of the Northern Hemisphere Ice Ages was obtained at nearly every site (Fig. 7a)(Fig. 7b). Sedimentation rates for four of the five North Atlantic drift sites are shown in (Fig. 8a)(Fig. 8b). Almost all of these sites have upper Pleistocene sedimentation rates greater than 10 cm/k.y., and Site 984 has upper Pliocene sedimentation rates in excess of 15 cm/k.y. It is clear from many of the records collected (e.g., Fig. 3) that detailed sampling will allow us to investigate changes in surface- and deep-water chemistry and sediment lithology on the time scale of hundreds to thousands of years.

The drift sites are interesting for two reasons. First, these sequences will provide information about the chemistry of water masses in these regions through time by isotopic and trace element analyses. Second, we will also be able to infer paleocurrent velocities through sedimentological analyses. Such investigations will allow us to test the response of thermohaline circulation to climate changes on many time scales and in many different climate regimes.

At all the sites, high-resolution continuous measurements of key lithologic parameters were made, such as magnetic susceptibility and spectral reflectance (Fig. 9). Such data allowed development of composite sequences and continuous time series of all parameters with no significant gaps in the triple APC sections. Ground-truthing the cause of variation in these nonintrusively measured parameters will be a high priority for initial shore-based studies. For instance, it appears that spectral reflectance may be a good indicator of carbonate percentage in the sedimentary sections (Fig. 10).

In order to understand the transformation of the Earth's climate system into an ice age world during the Neogene, it is important to identify where and why ice sheets started to form. Data on the inception, variability, and dynamics of these ice masses needs to be assessed for each ice sheet individually in order to understand which areas are the most sensitive to early ice sheet growth. For example, when did glaciation shift from mountain and fjord style glaciation to full fledged ice sheets, and when did marine-based ice sheets begin to extend to the outer continental shelf? To obtain this information, we cored Sites 986 and 987 close to the Svalbard and East Greenland margins, respectively. These last two sites of Leg 162 were planned in order to core continental margin sediments proximal to major Northern Hemisphere ice sheets; namely, the Barents/ Svalbard Ice Sheet in the European Arctic and the Greenland Ice Sheet. With these sequences we will be able to document and date the main phases of glaciation of the respective ice sheets as well as ground-truth the seismic network used to map the main glacial sequences on the margins. The successful coring of deep holes at both locations was a major achievement of the leg. Shipboard analyses document very different evolutionary histories of the two ice sheets, and provide new insight into similarities in the dynamics of glacial deposition between the two sites. At Site 987, glacial deposits exist throughout the sediment section, suggesting continuous glaciation on Greenland since the late Miocene, with major ice sheet expansion and deposition in the early and late Pliocene. The Barents/Svalbard Ice Sheet history appears to be much younger, probably starting in the late Pliocene with major expansion to the shelf break occurring in the Pleistocene.

Finally, at almost every site an interesting discovery was made from the pore-water profiles (Fig. 11). A downhole decrease in interstitial sulfate was observed at all sites. This decrease appears to be related to sedimentation rate; that is, greater and more rapid depletions of dissolved sulfate are observed at faster rates of deposition (Fig. 11A). This decrease may occur because fluxes of organic matter may be greater at higher sedimentation rates or because rapidly deposited sediments may restrict diffusive communication with overlying seawater to relatively shallow depths within the sediment. Besides the reduction of organic matter, the other important process which controls pore-water geochemistry is the alteration of basement rock as well as volcanic material within the sediment column. The degree of depletion in the Mg2+ profiles at these sites reflects the age and nature of the basement and the proximity of the site to a volcanic source (Fig. 11B). Sites 980, 981, and 982 exhibit the smallest Mg2+ depletions, reflecting the influence of a rifted continental block (i.e., Rockall Plateau) and the great distance from a volcanic source. The remaining sites are all located on oceanic basement and many receive significant inputs of volcanogenic sediment from Iceland. High heat flow at Sites 907 (121°C/km) and 986 (152°C/km) may also contribute to the extent of Mg2+ depletion by accelerating reaction rates between interstitial waters and basement (and/or sedimentary volcanic material).

Several of the most unexpected geochemical results are reflected in the dissolved chloride profiles (Fig. 11C). Dissolved chloride usually behaves conservatively in sediment pore waters, but three of the Leg 162 sites display downhole depletions in chloride concentrations (985, 986, and 987), four show little to no change (907B, 980, 981, and 983), and two sites record downhole increases in chloride (982, 984). Several processes were considered to explain such anomalous chloride behavior: (1) decomposition of methane hydrates (e.g., <400 mbsf at Sites 982 and 985); (2) hydration (Site 982) or dehydration (e.g., >400 mbsf in Site 982) of clay minerals; and (3) variable paleosalinity of the ocean (Sites 982 and 985). Additional shore-based work is necessary to test these hypotheses put forth to explain the enigmatic chloride anomalies at Leg 162 sites.

To Sites 980/981

162 Table of Contents

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