Early to middle Eocene history of the Arctic Ocean from Nd‐Sr isotopes in fossil fish debris, Lomonosov Ridge

Abstract

[1] Strontium and neodymium radiogenic isotope ratios in early to middle Eocene fossil fish debris (ichthyoliths) from Lomonosov Ridge (Integrated Ocean Drilling Program Expedition 302) help constrain water mass compositions in the Eocene Arctic Ocean between $55 and $45 Ma. The inferred paleodepositional setting was a shallow, offshore marine to marginal marine environment with limited connections to surrounding ocean basins. The new data demonstrate that sources of Nd and Sr in fish debris were distinct from each other, consistent with a salinity-stratified water column above Lomonosov Ridge in the Eocene. The

Paleoenvironmental implications of Cenozoic sediments from the central Arctic Ocean (IODP Expedition 302) using inorganic geochemistry

“…This induced Mn oxide formation, followed by transformation to Mn carbonates in otherwise anoxic sediments with significant enrichments of trace metals indicative for anoxic to euxinic bottom waters [e.g., Huckriede and Meischner, 1996;Emeis et al, 1998;Sternbeck et al, 2000;Sohlenius et al, 2001;Neumann et al, 2002]. Episodic, but short-termed flushing events of the deep Arctic Ocean during Eocene times with saline marine waters from the Atlantic are supported by Nd isotope data from fish bones [Waddell and Moore, 2008;Gleason et al, 2009] and by silicofossil preservation [Stickley et al, 2008]. Also a more detailed look at the records of generally enriched trace elements of Co, Cu, Fe, Mo, Ni, V and Zn reveals considerable variability (Figures 6 and 7) that is probably related to rapid changes in bottom water redox conditions.…”

“…The highly variable P/Al record (Figure 4b) also indicates redox fluctuations at the seafloor, as P is regenerated from sediments under anoxic conditions, while oxic conditions induce its preservation in the sediment as authigenic apatite [e.g., Filippelli, 1997;Delaney, 1998;Slomp and Van Cappellen, 2007]. Microfossil associations in ACEX unit D provide evidence for brackish to freshwater conditions, and the proposed shallow water depth on CLR [Moore and the Expedition 302 Scientists, 2006; Gleason et al, 2009] was probably in the same order of magnitude as the deep basins of the Baltic Sea (maximum of ∼450 m) In summary, geochemical data as well as previous paleoenvironmental considerations suggest a "Baltic Sea-type" rather than a "Black Sea-type" environment on CLR in the lower to middle Eocene.…”

“…The deposits are mostly of dark color [Backman et al, 2006], finely laminated, rich in pyrite and in TOC (1-6 wt% [Stein et al, 2006]). Freshwater chrysophyte cysts [Stickley et al, 2008], large amounts of microspore clusters (massulae) from the free-floating freshwater fern Azolla [Backman et al, 2006;Brinkhuis et al, 2006;Speelman et al, 2009], and isotope data from fish bones [Waddell and Moore, 2008;Gleason et al, 2009] indicate an episodically brackish to fresh surface water layer in the Middle Eocene Arctic Ocean. This is in line with the occurrence of stable seasonal sea ice by approximately 47 Ma (260 mcd), as recently proven by Stickley et al [2009].…”

“…Following the generally accepted idea of biogenic barite formation, a minimum water depth of 1000 m is required for the establishment of the sedimentary biogenic barite signal. Probably the Middle Eocene CLR was situated at shallower water depths [Moore and the Expedition 302 Scientists, 2006; Gleason et al, 2009], and biogenic barite simply did not form. Alternatively, biogenic barite was deposited, but diagenetically dissolved later on under sulfate-depleted conditions.…”

Clay mineralogy, strontium and neodymium isotope ratios in the sediments of two High Arctic catchments (Svalbard)

“…The "r" values are the Sr / Nd ratio of the "East" source divided by the Sr / Nd ratio of the "West" source. The star indicates the isotopic composition of Eocene seawater (Gleason et al, 2009, see text for details).…”