Tracing subarctic Pacific water masses with benthic foraminiferal stable isotopes during the LGM and late Pleistocene


Cook, Mea S.; Ravelo, A. Christina; Mix, Alan; Nesbitt, Ian M.; Miller, Nari V.

Deep-Sea Research Part II - Topical Studies in Oceanography (2016)

doi: 10.1016/j.dsr2.2016.02.006



As the largest ocean basin, the Pacific helps to set the global climate state, since its circulation affects mean ocean properties, air-sea partitioning of carbon dioxide, and the distribution of global oceanic poleward heat transport. There is evidence that during the Last Glacial Maximum (LGM) the subarctic Pacific contained a better-ventilated, relatively fresh intermediate water mass above ~2000 m that may have formed locally. The source and spatial extent of this water mass is not known, nor do we know how formation of this water mass varied during Pleistocene glaciations with different orbital and ice sheet boundary conditions. Here we present a 0.5 My multi-species benthic stable isotope record from Site U1345 (1008 m) on the northern Bering slope and a 1.0 My record from U1339 (1868 m) from the Umnak Plateau in the southeastern basin. We find that the relatively well-ventilated low-δ18O intermediate water reaches 1000 m in the Bering Sea during MIS2, but that the hydrographic divide between this water mass and poorly-ventilated deep water was shallower than 1000 m for earlier glaciations. We also compare Bering Sea piston core and IODP Expedition 323 Uvigerina data from the Holocene and LGM with the modern hydrography, and to previously published profiles from the Okhotsk Sea and Emperor Seamounts. We find that the carbon and oxygen stable isotope signatures of well-ventilated water in the Bering and Okhotsk Seas are distinct, suggesting that there may have been intermediate water formation in both basins during the LGM.

Plain-text abstract

Water masses in the Pacific Ocean, the largest ocean basin on Earth, help to regulate the planet's climate by storing carbon dioxide and by distributing heat from the equator to the poles. During the Last Glacial Maximum (~23 thousand years ago) the far-northern Pacific may have contained a fresh, well ventilated water mass that formed nearby (in what is now Coastal Alaska). Ventilation of a water mass is significant because it means that water mass is directly connected to the atmospheric composition of oxygen, carbon dioxide, and other chemicals, and therefore takes on attributes of the atmosphere at the time of its formation. The size and distribution of this water mass is not known, and it is not known how the formation of this water mass varied with different planetary orbit or nearby ice sheet conditions. The nature of this type of water mass can be investigated by examining the chemical properties of the shells of certain species of microscopic plankton found in the subsurface sediment of the seafloor. In this paper we present a 500,000 year record of the chemistry of those plankton shells from the northern Bering Sea slope, and a 1,000,000 year record from the Unmak Plateau in the Southerastern basin of the Bering Sea. The chamistry of the shells can also tell us the ratio of heavy oxygen to light oxygen (called the δ18O ratio) in the water at the time those species were alive. This δ18O ratio can be an indicator of whether a particular water mass came from an ice sheet or not, because the heavy oxygen does not fall on the ice sheets in as great an abundance as light oxygen. (This process is called oxygen isotope fractionation.) We find that the water mass discussed above reached 1000 meters depth during the period of 29,000-57,000 years ago (during the height of the last glacial period), but did not reach 1000 meters depth during earlier glaciations. This may indicate that the size of the ice sheet(s) contributing to this water mass's formation was larger than in previous ice ages. We compare our results to plankton chemistry studies from elsewhere in the North Pacific (Sea of Okhotsk and Emperor Seamounts) and find that the water mass in the Bering Sea was distinct, meaning that there may have been distinct water masses that developed in both basins during the last glacial period.