Two proximal, high-resolution records of foraminiferal fragmentation and their implications for changes in dissolution

摘要

Foraminiferal test fragmentation (150-250 μm-fraction) was studied at ～ 3-6 kyr resolution in ODP Sites 925 and 926 on Ceara Rise (～3000 and 3600m water depth) from ～6.34 to 5.0 Ma. This study investigates the driving forces behind changes in these two records and the potential of this proxy as an indicator of dissolution at depths above the lysocline. The records of fragmentation at both sites display variability concentrated in the precession band and the 125 and 96 kyr cycles of eccentricity superimposed on a low-frequency component, reminiscent of the 400 kyr cycle of eccentricity. Changes in terrigenous flux appear responsible for the manifestation of orbitally driven fragmentation susceptibility at the higher frequencies. The low-frequency trend is evident in both records of fragmentation, but marked by strong increases at the deeper site focusing on a dissolution-horizon suggestive of a supralysocline. Low coherence and shifting phases confound observed spectral power at 400 kyr, suggesting a mix of influences possibly unrelated to orbital variability. Proxies providing detail on changes in the sedimentary carbonate content reveal changes independent of the observed amplitude variability in the records of fragmentation. Low levels of carbonate accumulation and weight % sand up to 6.12 Ma suggest notable carbonate loss coincident with the first interval of increased fragmentation that may be influenced by a high surface to bottom δ~(13)C gradient in addition to other factors. Changes in carbonate content at Site 929 on Ceara Rise (4356m) reveal the same pattern of low-frequency carbonate dissolution as the records of fragmentation, thus identifying changes in depth of the carbonate compensation depth (CCD) as a driving force behind changes in carbonate preservation at shallower depths. Planktic foraminiferal test fragmentation appears to be a good proxy for pulsing of Antarctic Bottom Water (AABW) versus North Atlantic Deep Water (NADW) production at depths near and above the chemical lysocline (CL, ～80% carbonate content).