Abstract:
During Leg 198 (Shatsky Rise, north west Pacific) of the Ocean Drilling Program, Holes 1207B and 1213B were logged using a variety of downhole geophysical tools. Due to the poor recovery of Aptian-Albian (Lower Cretaceous) sediments, wireline logs provide us with the best method of investigating long-term lithologic and paleoceanographic change on Shatsky Rise during this period of time. Integration of the geophysical logs with limited sedimentological, biostratigraphic and physical properties data allows us to propose an Aptian-Albian depositional history for Shatsky Rise. At both sites, the lower Aptian is characterized by high resistivity and low porosity values. Within this unit is an interval with very high gamma ray values that corresponds to shale rich in organic-carbon that is also present in the recovered core. This shale represents the lithologic expression of the early Aptian Ocean Anoxic Event (OAE). Compared to the lower Aptian, the middle to upper Aptian has relatively low resistivity and high porosity values. In the lower Albian, resistivity returns to relatively high values and porosity to low values. By considering the discrete physical properties of Cretaceous pelagic chalks, limestones, porcellanites, cherts and claystones, it is possible to interpret the synchronous changes in the downhole logs from Sites 1207 and 1213 in terms of changing lithology. We suggest that the high resistivity units in the lower Aptian and the lower Albian are the result of an increased flux in biosiliceous material to the sea-floor resulting in an increase in the proportion of chert and porcellanite relative to chalk. Given that Shatsky Rise maintained a constant paleolatitude (straddling the equator) during the Aptian-Albian, we suggest that these periods of increased biosiliceous production were caused by intensification of equatorial upwelling, related to increased global warmth at these times. These proposed periods of increased upwelling are synchronous with OAE1a (early Aptian) and OAE1b (early Albian), the latter of which appears to have not left any record of increased organic-carbon burial in the Pacific basin. We discuss the implications of the proposed physical processes for ocean chemistry and paleoceanographic change, focusing on the generation (or not) of Oceanic Anoxic Events in the Pacific Ocean.