Geochemistry, grain-size distribution and smear slide abundances at DSDP Holes 68-501, 69-504 and 69-505

Rapidly deposited Miocene and younger siliceous and calcareous sediments at Sites 504 and 505 in the central Panama Basin near the eastern end of the equatorial high productivity belt exhibit marked lithologic variation with increasing depth. At both sites, the sediments are particularly calcareous just above the basaltic basement and were deposited at the highest accumulation rates. Sediments distinctly lower in carbonate and richer in clayey and siliceous components were deposited in the late Pliocene, a period characterized by lower accumulation rates. The sediments became somewhat more calcareous again in the Pleistocene, when accumulation rates rose again. Accumulation rates at Site 505, which is 77 km north of Site 504, have been enhanced by redeposition from surrounding basement highs, but the patterns of the accumulation rates at the two sites are similar. The lithologic variations reflect the sensitive interplay of crustal subsidence, fluctuations in the calcite compensation depth, and changes in the geographic location or the quantity of surface-water productivity. Computations using the bulk sediment composition of samples from Site 504 show that the rate of accumulation of biogenic silica increased nearly fourfold during the upper Pliocene, despite the drop in carbonate and overall accumulation rates. This could be explained by the shoaling of the calcite compensation depth beneath a zone of very high surface productivity, which would increase the carbonate dissolution in the water column but leave biogenic opal comparatively unaffected. Chemical data for the sediments also show a slight enrichment in Mn, Fe, Cu, Zn, and other trace metals at about this time, reflecting increases in both hydrogenous and metalliferous ("hydrothermal") components. The biological fixation of seawater sulfate as pyrite, in combination with the high sediment accumulation rates, probably contributed to the enrichment of some of the trace metals. Chemical data also show that several of the Pleistocene and one of the lower Pliocene ash beds are rhyolitic in composition and represent material derived from the South American eruption of ignimbrites. In all beds except these, however, bioturbation has dispersed ash to uniform and low abundances in the sediments. Sites 504 and 505 are in contrasting locations of high and low heat flow, respectively. Temperatures near 50°C just above basalts at Site 504 have promoted the formation of chert bands intercalated with limestone in the 30 meters above the basalts and have accelerated the induration of nearly 90 meters of sediments above the cherts to chalk. The thickness of the sediments at Site 505 is similar, but the sediments remain oozes nearly down to basement, where temperatures are only about 9°C