Down Core Fidelity of Authigenic Oxyhydroxide Coatings: Tracing Paleogene Water Mass Evolution

Abstract:

Marine Fe-Mn oxyhydroxides may provide an opportunity to reconstruct the dissolved metal composition of past oceans, as these phases often precipitate directly from seawater. Recent work has demonstrated Fe-Mn oxyhydroxides chemically leached from core top sediments preserve the Nd isotopic composition of seawater. However, one challenge to applying this tracer of terrestrial weathering and water mass movements down core is that oxyhydroxide phases are readily reduced as dissolved oxygen levels decrease in sediment porewaters. The initial step we take to evaluate downcore preservation of the oxyhydroxide Nd isotope composition is to target sites in the South Pacific Gyre, where oxygen has been shown to penetrate the entire sediment column.

Extracted Fe-Mn oxyhydroxide and published fossil fish debris Nd data at DSDP Site 596 and IODP Site 1370 record the same values downcore (i.e. ε_Nd(t) = -7.4 to -7.8). Previous studies indicate that Paleogene circulation consisted of two distinct systems in the Pacific and Atlantic. Sites 596 and 1370 are ideally situated to observe the evolution of the isotopic composition of bottom water as it moves out of the Southern Ocean during the transition from two separate ocean systems to modern global overturning circulation. At Site 596 the Nd isotopes were relatively constant from ~70 to 38 Ma indicating a stable water mass influence over that time period. From 38 Ma until 10 Ma they varied significantly with a notable swing measured both in the fish teeth and oxide coatings from -7.3 ε_Nd(t)  at 20.7 Ma to -3.4 ε_Nd(t) at 18.4 Ma. At Site 1370, the more southern site, there is a general trend towards less radiogenic values from -5.3 ε_Nd(t) at ~60 Ma to -7.3 at ~10 Ma, and no corresponding radiogenic swing. This indicates a potential divergence of water mass sources between the sites and the influence of a less radiogenic (North Atlantic or Ross Sea) source mixing into the Southern Ocean bottom water.