INVESTIGATING MINERAL AND GEOCHEMICAL MANIFESTATIONS OF RIDGE FLANK ADVECTION ON ABYSSAL HILL TERRAIN NEAR THE EAST PACIFIC RISE

In order to understand mid-ocean ridge advective fluid flow processes, more must be known about the role of young (0.1-4 Ma) ridge flanks. In particular, the role of abyssal hills in ridge flank fluid flow is essential for a more complete knowledge of ridge flank processes. In recent years, the discovery of a novel microbial community on an abyssal hill fault scarp as well as observed sedimentary mounds on top of the same abyssal hill have raised new questions regarding the presence of fluid advection through young ridge flanks. To address questions regarding abyssal hills acting as conduits for ridge flank fluid discharge, a suite of sediment cores was collected from ~0.2-0.5 Ma abyssal hilltops on the flanks of the fast-spreading East Pacific Rise between 9°27' and 9°45'N. By analyzing a correlating set of sediment and interstitial fluid samples, we are attempting to ascertain if ridge flank advective fluid flow is facilitated by the expulsion of hydrothermally altered fluids through abyssal hill sediment.

Observations of recovered cores reveal distinct differences in core stratigraphy. Some cores are characterized by a thin (~20 cm) layer of dark brown sediment underlain by green colored sediment (Type I), while the remaining cores are dominated by thick, disturbed layers of brown, tan, and olive colored sediments (Type II). Interstitial fluids collected from Type I cores appear to show lithological control on fluid Mg and Ca concentration, which may indicate that these cores represent background diagenesis of ocean sediment near ridge crests. Interstitial fluids from Type II cores contain Mg and Ca signals that may be hydrothermally influenced and appear to correlate with their stratigraphy.

When compared with abyssal hill fault scarp height, Type I cores were all collected on hills with scarp heights of less than 100m (median height = 80m), while Type II cores were collected on hills with scarp heights over 100m (median height = 150m). This suggests that taller abyssal hills have deeper and more active faults that periodically crack the crustal fluid reservoir, resulting in increased hydrothermal flow. In addition core type does not correlate with geographic distance from the ridge crest, implying that in this case, distance from the ridge crest does not govern the presence or robustness of hydrothermal advective flow.