Isotopic composition of oxygen, carbon, and sulfur in interstitial water and cores at DSDP Leg 59 Holes

Comprehensive isotopic studies based on data from the Deep Sea Drilling Project have elucidated numerous details of the low- and high-temperature mechanisms of interaction between water and rocks of ocean crustal seismic Layers 1 and 2. These isotopic studies have also identified climatic changes during the Meso-Cenozoic history of oceans. Data on the abundance and isotopic composition of sulfur in the sedimentary layer as well as in rocks of the volcanic basement are more fragmentary than are oxygen and carbon data. In this chapter we specifically concentrate upon isotopic data related to specific features of the mechanisms of low-temperature interaction of water with sedimentary and volcanogenic rocks. The Leg 59 data provide a good opportunity for such lithologic and isotopic studies, because almost 600 meters of basalt flows and sills interbedded with tuffs and volcaniclastic breccias were cored during the drilling of Hole 448A. Moreover, rocks supposedly exposed to hydrothermal alteration play an important role at the deepest horizons of that mass. Sulfur isotopic studies of the character of possible biogenic processes of sulfate reduction in sediments are another focus, as well as the nature and origin of sulfide mineralization in Layer-2 rocks of remnant island arcs. Finally, oxygen and carbon istopic analyses of biogenic carbonates in the cores also enabled us to investigate the effects of changing climatic conditions during the Cenozoic. These results are compared with previous data from adjacent regions of the Pacific Ocean. Thus this chapter describes results of isotopic analyses of: oxygen and sulfur of interstitial water; oxygen and carbon of sedimentary carbonates and of calcite intercalations and inclusions in tuffs and volcaniclastic breccias interbedded with basalt flows; and sulfur of sulfides in these rocks.

Velocities and elastic moduli of basalts from Pacific DSDP sites

Compressional (Vp) and shear (Vs) wave velocities have been measured to 10 kb in 32 cores of basalt from 14 Pacific sites of the Deep Sea Drilling Project. Both Vp and V s show wide ranges (3.70 to 6.38 km/sec for Vp and 1.77 to 3.40 km/sec for V s at 0.5 kb) which are linearly related to density and sea floor age, confirming earlier findings by Christensen and Salisbury of decreasing velocity with progressive submarine weathering based on studies of basalts from five sites in the Atlantic. Combined Pacific and Atlantic data give rates of decreasing velocity of -1.89 and -1.35 km/sec per 100 my for Vp and Vs respectively. New analyses of oceanic seismic refraction data indicate a decrease in layer 2 velocities with age similar to that observed in the laboratory, suggesting that weathering penetrates to several hundred meters in many regions and is largely responsible for the extreme range and variability of layer 2 refraction velocities.

Chert intervals in DSDP and ODP sites (Table 1)

Radiolarian cherts in the Tethyan realm of Jurassic age were recently interpreted as resulting from high biosiliceous productivity along upwelling zones in subequatorial paleolatitudes the locations of which were confirmed by revised paleomagnetic estimates. However, the widespread occurrence of cherts in the Eocene suggests that cherts may not always be reliable proxies of latitude and upwelling zones. In a new survey of the global spatio-temporal distribution of Cenozoic cherts in Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment cores, we found that cherts occur most frequently in the Paleocene and early Eocene, with a peak in occurrences at ~50 Ma that is coincident with the time of highest bottom water temperatures of the early Eocene climatic optimum (EECO) when the global ocean was presumably characterized by reduced upwelling efficiency and biosiliceous productivity. Cherts occur less commonly during the subsequent Eocene global cooling trend. Primary paleoclimatic factors rather than secondary diagenetic processes seem therefore to control chert formation. This timing of peak Eocene chert occurrence, which is supported by detailed stratigraphic correlations, contradicts currently accepted models that involve an initial loading of large amounts of dissolved silica from enhanced weathering and/or volcanism in a supposedly sluggish ocean of the EECO, followed during the subsequent middle Eocene global cooling by more vigorous oceanic circulation and consequent upwelling that made this silica reservoir available for enhanced biosilicification, with the formation of chert as a result of biosilica transformation during diagenesis. Instead, we suggest that basin-basin fractionation by deep-sea circulation could have raised the concentration of EECO dissolved silica especially in the North Atlantic, where an alternative mode of silica burial involving widespread direct precipitation and/or absorption of silica by clay minerals could have been operative in order to maintain balance between silica input and output during the upwelling-deficient conditions of the EECO. Cherts may therefore not always be proxies of biosiliceous productivity associated with latitudinally focused upwelling zones.

Opal concentration of ODP Leg 178 holes

Neogene to Quaternary records of biogenic opal contents and opal accumulation rates are presented for Sites 1095, 1096, and 1101, which were drilled during Ocean Drilling Program Leg 178 in the Bellingshausen Sea, a marginal sea in the eastern Pacific sector of the Southern Ocean. The opal records in the drift sediments on the continental rise west of the Antarctic Peninsula provide signals of paleoproductivity, although they are influenced by dissolution in the water column and the sediment column. Opal contents at Sites 1095, 1096, and 1101 show similar long-term trends through the Neogene and Quaternary, whereas the opal accumulation rates exhibit marked discrepancies, which are caused by local differences in opal preservation linked to local variations of bottom current-induced supply of lithogenic detritus. We used a regression describing the relationship between opal preservation and sedimentation rate to extract the signal of primary opal deposition on the seafloor in the Bellingshausen Sea from the opal accumulation in the drift deposits. On long-term timescales, the reconstructed opal depositional rates show patterns similar to those of the opal contents and a much better coherency between the different locations on the Antarctic Peninsula continental rise. Therefore, the estimated opal depositional rates are suggested to represent a suitable proxy for paleoproductivity in the drift setting of the Bellingshausen Sea. Supposing that the sea-ice coverage within the Antarctic Zone was the main factor controlling biological productivity in the Bellingshausen Sea, and thus the estimated opal depositional rates on the continental rise, we reconstructed paleoceanographic long-term changes during the Neogene and Quaternary considering the climatic control on regional and global scales. Slightly enhanced opal depositional rates during the late Miocene are interpreted to indicate warmer climatic conditions in the vicinity of the Antarctic Peninsula than at present. The contribution of heat from the Northern Component Water (NCW) into the Southern Ocean seems only to have played a subordinate role during that time. High opal depositional rates during the early Pliocene document a strong reduction of sea-ice coverage and relatively warm climatic conditions in the Bellingshausen Sea. The early onset of the Pliocene warmth points to a positive feedback of regional Antarctic climate on the global thermohaline circulation. A decrease of opal deposition between 3.1 and 1.8 Ma likely reflects sea-ice expansion in response to reduced NCW flow, caused by the onset and intensification of Northern Hemisphere glaciation. Throughout the Quaternary, a relatively constant level of opal depositional rates in the Bellingshausen Sea indicates stable climatic conditions in the Antarctic Peninsula area.

Pliocene to Pleistocene planktonic foraminiferal datum levels in the North Pacific

Continuous late Neogene planktonic foraminiferal records have been studied in the deep-sea cores of DSDP Sites 173, 310, and 296 across mid-latitudes of the North Pacific. These three sites have been correlated on the basis of planktonic foraminiferal events and major paleoclimatic/paleoceanographic intervals and tied to diatom, radiolarian, and nannofossil datum levels, and paleomagnetic and isotopic stratigraphy. Ten planktonic foraminiferal datum levels have been recognized within these Pliocene to Pleistocene sections; two of these are recognizable within the Pleistocene and eight within the Pliocene. Six planktonic foraminiferal zones are proposed which combined with the foraminiferal datum levels provide a high resolution biostratigraphic correlation for the mid-latitudes of the North Pacific.