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doi:10.2204/iodp.proc.342.201.2017

Discussion

There is good agreement between paleontological and sedimentological methods for assessing the abundance of siliceous microfossils in Expedition 342 sediments. It is highly unlikely that biogenic opal makes a significant contribution to sediment composition in intervals where siliceous microfossils were not recorded.

There is a consistent pattern in the distribution of siliceous microfossil-rich intervals in Expedition 342 sediment cores (Fig. F10). Siliceous microfossils are common in the Pleistocene in the two deepest sites on southeast Newfoundland ridge (Sites U1409 and U1410; >3300 m water depth) but are otherwise rare in the upper Neogene. Both diatoms and radiolarians are abundant and well preserved in lower Miocene and upper Oligocene drift sediments on the J-Anomaly Ridge (Sites U1404, U1405, and U1406) but are absent from sediments of this age on southeast Newfoundland ridge. Significantly, siliceous microfossils are absent from all sites for ~10 My spanning the Eocene–Oligocene transition, from lower Oligocene to uppermost middle Eocene. Radiolarians are abundant from middle Eocene to upper lower Eocene sediments at several sites, tending to be more persistent in the deeper J-Anomaly Ridge sites (U1403 and U1404) than in shallower sites on J-Anomaly Ridge (U1406 and U1407) and all sites on southeast Newfoundland ridge (U1408, U1409, and U1410). Radiolarians are poorly preserved in a chert-rich interval in the lower Eocene and spanning the Paleocene/Eocene boundary, notably at Sites U1403 and U1407. In contrast, very well preserved, abundant, and diverse radiolarians occur in the upper Paleocene calcareous ooze, which is present at Sites U1406–U1409. Indeed, all sites that penetrated the Paleocene in this region encountered a radiolarian-rich interval that encompassed portions of Zones RP6 and RP7, an interval that is also rich in radiolarians at Deep Sea Drilling Project (DSDP) Site 384 (Nishimura, 1992). Preservation is excellent at the shallower sites, where radiolarians occur within the Paleocene nannofossil ooze, but is variable at Site U1403, where sediments are more siliceous.

Diatoms are rare in the lower Paleogene and are restricted to intervals where radiolarians are abundant in the middle to lower Eocene and upper Paleocene. A similar record of siliceous microfossil abundance was recorded at DSDP Site 384, which was drilled on J-Anomaly Ridge at a water depth of 3909 m (Tucholke, Vogt, et al., 1979), where radiolarians are abundant and well preserved from middle to upper lower Eocene (Zones RP11 through RP9) and from upper to lower Paleocene (Zones RP7 through RP6a; Nishimura, 1992). The Paleocene–Eocene transition is not preserved at this site.

This distribution pattern indicates that siliceous microfossil accumulation is greatest during the two main phases of drift sedimentation, particularly the Miocene–Oligocene drifts of J-Anomaly Ridge but also in the middle Eocene drifts on southeast Newfoundland ridge and at Sites U1403 and U1404. Radiolarians tend to occur more regularly at deep-water sites (U1403 and U1404) in the Eocene. In the shallower sites, there is a weak trend toward shorter intervals of abundant radiolarian with shoaling. In the Eocene, abundant radiolarians occur over ~3.5 My at Site U1406 (3814 m), over ~4 My at Sites U1409 (3502 m) and U1410 (3387 m), over ~2 My at Site U1407 (3074 m), and over ~0.5 My at Site U1408 (3022 m).

In addition to this depth relationship, it also seems likely that the distribution of siliceous microfossil assemblages may be related to the evolution of northwest Atlantic circulation patterns. Siliceous sediments are restricted to the northeast margin of the southeast Newfoundland ridge in the Pleistocene, suggesting that the ridge may mark the oceanographic boundary between Subantarctic and temperate waters and perhaps the southern limit of the Labrador Current at this site (Bohrmann and Stein, 1989). The two main intervals of abundant siliceous microfossils correspond with periods of relatively warm climate (i.e., the Oligocene–Miocene transition and the lower to middle Eocene) (Zachos et al. 2008). These time intervals may have been associated with greater mixing of subantarctic and subtropical waters, which may have promoted biosiliceous productivity at these sites. This scenario is consistent with the low-latitude affinities of the radiolarian assemblages (C.J. Hollis et al., unpubl. data; Jin et al., 2015) and may explain why a time of relatively cooler climatic conditions, from the early Oligocene to late Eocene, is not associated with enhanced biosiliceous productivity in this midlatitude setting. The scarcity of siliceous microfossils at Site U1411 supports this hypothesis. An expanded succession of drift sediments spans the Eocene/Oligocene boundary. Therefore, high burial rates alone cannot account for preservation of siliceous microfossils.

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