Variation in bioturbation with water depth on marine slopes: a study on the Little Bahamas Bank

Reconstructing the paleoceanography of intermediate-depth waters is dependent on sedimentary records preserved on marine slopes. But knowledge of bioturbation processes in such slope environments is much poorer than for the deep sea. In this study, Pb profiles were measured for the upper approximate to 20 cm of sediment from five box cores taken on the slopes of the Bahamas in water depths ranging from 400 to 1500 m. These were compared with data from the Bahamas bank tops and from elsewhere to assess the rate and depth of mixing in the slope: environment. Excess Pb-210 inventories are too high to explain by water-column decay of Ra-226 and are derived largely from atmospheric fallout. Down-core Pb-210(xs) profiles show an upper zone where Pb-210(xs) activities decrease exponentially with depth suggesting uniform mixing, and a lower zone with patchy Pb-210 values reflecting isolated burrows. In contrast to bank-top sediment which shows Pb-210(xs) mixing to depths of approximate to 40 cm and mixing rates of > 1000 cm(2)/kyr, mixing of Pb-210 on the slopes penetrates to only approximate to 8 cm with mixing rates of < 100 cm(2)/kyr. Some systematic variation of mixing parameters with water depth are seen on the slopes but these are subtle compared to the dramatic change between bank-top and slope:. This pattern is also seen in literature data from regions with relatively slow sedimentation rates and is fit reasonably well by the relationship: mixing rate = 6330(depth(-1.65)) [Soetaert, K., Herman, P.M.J., Middelburg, J.J., Help, C., deStigter, H.S., van Weering, T.C.E., Epping, E., Helder, W., 1996. Modelling Pb-210-derived mixing activity in ocean margin sediments: Diffusive versus nonlocal mixing. J. Mar. Res. 54, 1207-1227]. A simple model is used to assess the effects of mixing on typical Bahamian slope sediments which have deposition rates and compositions which vary dramatically in response to sea-level change. Proxy records such as delta(18)O are expected to be displaced by up to 0.8 kyr in bulk sediment. And individual sediment constituents may be moved by up to 2 kyr relative to one another. These mixing effects are reasonably small and may be safely ignored for some studies. But for high-resolution records, or studies where precise age control is important, bioturbation of slope sediments should be considered. (C) 1999 Elsevier Science B.V. All rights reserved.