IODP Proceedings    Volume contents     Search
iodp logo



The Lomonosov Ridge was rifted from the Siberian continental margin sometime near 57 Ma. Tilting and erosion of the rifted blocks ended by ~56 Ma when hemipelagic sediment began above the rifting unconformity. The subsidence model used here assumes that subsidence of the ridge began ~54 Ma through cooling that followed the rifting phase. This simple subsidence model is generally consistent with lithologic and benthic foraminifer data that indicate a relatively shallow water (neritic) environment of deposition on the Lomonosov Ridge up until ~54 Ma. Within the Paleogene section, suboxic conditions existed in ridge sediments as they subsided from ~200 to 350 m paleowater depth. Once the ridge reached paleowater depths greater than ~350 m, anoxic sediments, rich in organic carbon and well-preserved microfossils, began to accumulate. These sediments and the estimated water depths at which they were deposited are consistent with the presence of a strong oxygen minimum zone.

The presence of the large middle Eocene to lower Miocene hiatus in the sections recovered from the Lomonosov Ridge marks a transition from an underlying anoxic to suboxic Paleogene section to a more oxic Miocene and younger section and marks a profound change in both the lithologic character and the fossil content of the sediments. The character of the bedding just above the Eocene section strongly suggests that this hiatus is associated with relatively strong currents that eroded, reworked, and redeposited upper Eocene, Oligocene, and perhaps Miocene sediments. It is not known whether this hiatus is primarily associated with the subsidence of the ridge into a depth zone dominated by strong currents or with the tectonic control on connections between the Arctic and the North Atlantic. In either case, the cooling of the global climate through the middle and late Eocene and into the Oligocene undoubtedly is linked to these changes.

The fact that reworked dinocysts having a “Miocene aspect” appear within the gray, suboxic sediments just below the Miocene oxic section in lithostratigraphic Subnit 1/5 suggests that the shift to fully oxygenated deep waters in the Arctic did not take place till sometime in the Miocene. The earliest appearance of NCW in the deep North Atlantic occurred at 19–20 Ma (Wright and Miller 1996, Wright, 1998) and may mark the time that the Greenland-Faeroe Ridge had subsided to a depth that allowed a gradually more vigorous exchange between the Norwegian-Greenland Sea, the Arctic, and the North Atlantic.