Recent to Middle Miocene Diatom Productivity at Site 907, Iceland Plateau

Abstract

Surface water productivity at Ocean Drilling Program Site 907 has been assessed from total diatom abundance and Chaetoceros resting spore frequency. The sediments are divided into four intervals based on relative diatom abundance. Intervals 1 and 4 are mainly barren of diatoms. Interval 2 shows a highly fluctuating abundance pattern, whereas Interval 3 is a biosilicarich middle to late Miocene (13.7-10.1 Ma) sediment containing abundant diatoms dominated by Chaetoceros resting spores. Resting spores of Chaetoceros are classified according to their forms. The round to elongate spores with smooth surfaces or spines/ridges dominate the assemblage throughout the biosilica-rich interval. The transition from the diatom barren interval to the biosilica-rich interval at about 13.7 Ma is characterized by extremely high (100%-85%) Chaetoceros spore content. The late Miocene to late Pliocene (10.1-3.0 Ma) interval with highly variable diatom content is characterized with lower Chaetoceros percentages than in the biosilica-rich interval. We infer that the period between 13.7 and 10.1 Ma reflects high surface water productivity at Site 907. The high percentage of Chaetoceros resting spores at this interval suggests upwelling of nutrient-rich subsurface waters during this period.

A major change in North Atlantic deep water circulation 1.6 million years ago

“…The decrease in Atlantic overturning we observe here during interglacial stages is corroborated by studies of siliceous microfossils (e.g., Koç and Scherer, 1996;Stabell and Koç, 1996). These authors concluded based on the absence of production of silica in the surface waters of the GIN seas relative to the region south of Iceland (Koç and Flower, 1998;Koç et al, 1999) that a strong front must have existed near the Greenland-Scotland Ridge during most of the interglacials of the early to mid-Pleistocene (summarized by Raymo et al, 2004).…”

Response of marine palynomorphs to Neogene climate cooling in the Iceland Sea (ODP Hole 907A)

“…The Middle Miocene transition (14.5-13.0 Ma) Sedimentation at Site 907 commenced shortly after the MCO and high palynomorph concentration corroborates a high primary productivity , which has also been inferred from abundant diatoms and their resting spores after 14 Ma (Stabell and Koç, 1996). However, as protoperidiniacean dinocysts are usually abundant in upwelling areas (e.g.…”

“…However, as protoperidiniacean dinocysts are usually abundant in upwelling areas (e.g. Reichart and Brinkhuis, 2003;Holzwarth et al, 2010), their low abundance during this interval in the Iceland Sea does not support substantial upwelling as suggested by Stabell and Koç (1996). Since selective degradation is excluded due to light to dark brown colors of cyst walls and no evidence of oxidative damage of Barssidinium and Cristadinium specimens encountered, the high paleoproductivity likely resulted from increased nutrient input due to enhanced chemical weathering during a much warmer and humid climate (Wan et al, 2009;Ma et al, 2011;Pound et al, 2012).…”

“…Bordovskiy, 1965), although they should be interpreted cautiously due to low TOC contents of ≤0.5% (Stein and Macdonald, 2004). However, diatoms also show a distinctive shift from a continuously high to a more variable abundance pattern around 10 Ma (Stabell and Koç, 1996). Variable surface and deep-water circulation and progressive Late Miocene cooling has also been reported from the Norwegian Sea (Bohrmann et al, 1990;Ciesielski and Case, 1989;Locker and Martini, 1989), whereby the former is likely coupled to GSR sill depth variations (Poore et al, 2006).…”

“…Increased bulk accumulation rates at ODP Site 909 supposedly reflect intensification of water mass exchange through Fram Strait (Wolf-Welling et al, 1996;Winkler et al, 2002), which would have resulted in enhanced advection of comparatively cold, low saline, and presumably ice covered waters into the Iceland Sea, forcing the observed changes in the palynomorph assemblage. The increased advection of colder waters may have triggered the coeval collapse in accumulation of Chaetoceros resting spores reflecting a diminished diatom production (Stabell and Koç, 1996). Benthic δ 18 O values on Vøring Plateau indicate substantial deep water cooling at~11 Ma (Fronval and Jansen, 1996), and increased circum-Arctic IRD fluxes between 11.0 and 9.5 Ma (Schaeffer and Spiegler, 1986;Thiébault et al, 1989;Wolf and Thiede, 1991;Fronval and Jansen, 1996;Wolf-Welling et al, 1996;Helland and Holmes, 1997;Winkler et al, 2002) prove general cooling in the high northern latitudes.…”