| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 102 === At present the deep ocean carbon isotope minima (-0.2&;#8240;) is in the North Pacific, whereas during the Last Glacial Maximum (LGM) the minima (-0.9&;#8240;) was in the South Atlantic. Several previous studies pointed out that this shift was caused by additional freshly-formed waters in the North Pacific [Curry et al., 1988; Duplessy et al., 1988; Herguera et al., 1992; Keigwin, 1998], but Toggweiler et al. [2006] suggested the Pacific deep water was affected mainly by the deep water source from North Atlantic.
In northwestern Pacific, the previous studies covered only the history of deep-ocean circulation patterns within the past 35 kyr, although a few records on the Shatsky Rise discussed surface water mass circulations over the past 180 kyr. To extend our understanding of the northwestern Pacific deep water mass circulation, we analyzed carbon and oxygen isotopes (δ13C and δ18O) of benthic foraminifera Uvigerina peregrina (300-425 μm) of Hole ODP 1210A (32o13.4’N, 158o15.6’E; water depth 2573.6 m) cored from Shatsky Rise. An age model for the past 180 kyr was established by 14C dating data and U. peregrina δ18O curve correlated to the LR04 global benthic foraminifera δ18O stack [Lisiecki and Raymo, 2005].
The results of benthic foraminifera δ18O and δ13C of three cores (ODP 1210A、NGC102、S-2) on Shatsky Rise showed they have been in the same water mass, Circumpolar Deep Water (CDW), during the past 180 kyr. We compared eight δ13C records of the western Pacific and found they have similar δ13C patterns during the past 180 kyr, implying that the deep water below 2500 m in the western Pacific were influenced by the Deep Western Boundary Current (DWBC). Because the δ13C records of northwestern Pacific did not show higher δ13C values than that in the southwestern Pacific during the glacial periods, we consider that the Glacial North Atlantic Intermediate Water (GNAIW) did not influence the deep waters below 2500 m in the northwestern Pacific.
Our study point out that the δ13C records of 2500-4000 m deep water in western Pacific was composed by 60% NADW and 40% AABW modified with a reminerlization constant [Lisiecki, 2010]. As the water mass was transported from the southwest to northwest Pacific, more organic matters were decomposed and the δ13C of seawater decreased. Due to this remineralization, the adjusting constant in the formula for the western equatorial and northwestern Pacific δ13C is -0.5&;#8240;, and for the southwestern Pacific is -0.2&;#8240;. Besides, the transportation of deep waters from the southwestern Pacific into the Pacific Basin was faster in glacial times than in the interglacial times. This increase of flow rate of the deep water resulted in a smaller difference between the δ13C of the western equatorial/northwestern Pacific and the δ13C of the southwestern Pacific during the glacial times.
In summary, we consider the Pacific deep water was affected mainly by the deep water sources from Atlantic during the past 180 kyr. The shift of the deep ocean δ13C minima was not caused by additional freshly-formed waters in the North Pacific.
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