A New Stable Isotope Record From the Subantarctic Southeastern Pacific

Few stable isotope records exist from the southeast Pacific Ocean due to its remote location, low sedimentation rates, and shallow carbonate compensation depth (CCD). The CCD in the southeast Pacific, however, has been found to be unusually deep (~4750 m), thereby allowing for the preservation of Neogene carbonates at abyssal depths. Herein we present stable isotope results from MV0502-4JC, which was recently recovered from the subantarctic region of the Southwest Pacific (50°20S, 148°08W, 4286 m). Dating the record with radiolarian biostratigraphy, we have generated a benthic stable isotope record back to the Middle Miocene from Cibicidoides spp. and a planktic record from Globigerina bulloides into the Late Pliocene. These stable isotope records, in conjunction with carbonate contents and counts of ice-rafted debris (IRD) and manganese micronodules show the effect of global ice sheet build up on this poorly understood region. A prominent negative δ13C shift of ~1‰ (from ~1 to ~0.2‰) in the benthic record occurs at ~15.3 MBSF (Middle Miocene) as carbonate contents in the core decrease from ~80% to ~20%, and significant changes in the bottom water are indicated by changes in the dominant Cibicidoides spp. at the site from C. robertsonianus to C. wuellerstorfi. Trace amounts of IRD and abundant manganese micronodules also appear at the site at this time. A significant hiatus (from the Late Miocene to the Middle Pliocene) occurs somewhere between 11.4 and 9.8 MBSF. From 9.8 to 8 MBSF (Late Pliocene) benthic δ18O increases and δ13C decreases by ~1‰. Also, IRD increases, manganese micronodules decrease, and there is a dramatic increase in the preservation of planktic foraminifera at the site. A planktic δ13C shift of >1‰ (from 0 to 1‰) occurs at ~5 MBSF (Pleistocene) coincident with a 20% increase in carbonate concentration. After this interval, the variability in both benthic δ18O and δ13C and planktic δ18O increases significantly. We interpret the negative δ13C shift at 15.3 MBSF as the end of the Monterey Carbon Excursion (~13.5 Ma), and the decrease in carbonate observed in MV0502-4JC following this shift may correspond with a widespread hiatus (NH3) associated with the increased presence Antarctic Bottom Water (AABW) in the Pacific and shoaling of the lysocline. The significant hiatus in the Late Miocene-Early Pliocene may also indicate increased AABW formation and Antarctic Circumpolar Current strength during key periods of ice sheet expansion on Antarctica. The ~1‰ increase in benthic δ18O in the Late Pliocene is interpreted as the onset of Northern Hemisphere Glaciation, although the planktic δ18O record oddly does not show this trend. Increased lysocline depth during the Late Pliocene preceded the high (~30 cm kyr-1) sedimentation rates of the Early Pleistocene that may have resulted from reduced bottom water currents and/or increased productivity. The Late Pleistocene was associated with lower sedimentation rates perhaps associated with increased non-carbonate productivity and increased AABW production during glacial intervals.