The paleoceanography of the Bering Sea during the last glacial cycle

Abstract

In this thesis, I present high-resolution stable-isotope and planktonic-fauna records from Bering Sea sediment cores, spanning the time period from 50,000 years ago to the present. During Marine Isotope Stage 3 (MIS3) at 30-20 ky BP (kiloyears before present) in a core from 1467 m water depth near Umnak Plateau, there were episodic occurrences of diagenetic carbonate minerals with very low δ 13 C (−22.4 ), high δ 18 O (6.5 ), and high [Mg]/ [Ca], which seem associated with sulfate reduction of organic matter and possibly anaerobic oxidation of methane. The episodes lasted less than 1000 years and were spaced about 1000 years apart. During MIS3 at 55-20 ky BP in a core from 2209 m water depth on Bowers Ridge, N. pachyderma (s.) and Uvigerina δ 18 O and δ 13 C show no coherent variability on millennial time scales.Bering Sea sediments are dysoxic or laminated during the deglaciation. A high sedimentation rate core (200 cm/ky) from 1132 m on the Bering Slope is laminated during the Bølling warm phase,Ållerød warm phase, and early Holocene, where the ages of lithological transitions agree with the ages of those climate events in Greenland (GISP2) to well within the uncertainty of the age models. The subsurface distribution of radiocarbon was estimated from a compilation of published and unpublished North Pacific benthic-planktonic 14 C measurements (475-2700 m water depth). There was no consistent change in 14 C profiles between the present and the Last Glacial Maximum, Bølling-Ållerød, or the Younger Dryas cold phase. N. pachyderma (s.) δ 18 O in the Bering Slope core decreases rapidly (in less than 220 y) by 0.7-0.8 at the onset of the Bølling and the end of the Younger Dryas. These isotopic shifts are accompanied by transient decreases in the relative abundance of N. pachyderma (s.), suggesting that the isotopic events are transient warmings and sustained freshenings.

Assessing the ability of the 14C projection‐age method to constrain the circulation of the past in a 3‐D ocean model

“…An alternative way to compute projection ages would therefore be to use reconstructions of the tropical surface ocean Δ 14 C as applied by Skinner and Shackleton [2004] or the mean ocean surface Δ 14 C curve MARINE04 [ Hughen et al , 2004] which was chosen by Cook [2006]. This fundamental correction can be accomplished with the present state of knowledge but uncertainties concerning the specific reservoir‐age variations in the source regions remain.…”

Last Glacial Maximum to Holocene sea surface conditions at Umnak Plateau, Bering Sea, as inferred from diatom, alkenone, and stable isotope records

“… a Calendar ages calibrated using Calib version 5.0.2 and the Marine 04 calibration curve [ Hughen et al , 2004]. A total reservoir age of 800 years (ΔR = 400) was used to calculate calibrated ages [after Cook , 2006]. …”

“…A reservoir age of 800 years was used to be consistent with other published studies of these cores [e.g., Cook et al , 2005; Cook , 2006]. However, this value is relatively poorly constrained with estimates in the region ranging from 480 to 1100 years for total modern reservoir corrections [ Kuzmin et al , 2001; Dumond and Griffin , 2002; Kovanen and Easterbrook , 2002].…”

“…[Hughen et al, 2004]. A total reservoir age of 800 years (DR = 400) was used to calculate calibrated ages [after Cook, 2006]. counted according to the methods of Schrader and Gersonde [1978].…”