Sediment focusing in the central equatorial Pacific Ocean

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At four sites in the central equatorial Pacific Ocean the flux of extraterrestrial He-3, determined using the excess Th-230 profiling method, is 8 x 10(-13) cm(3) STP cm(-2) ka(-1). This supply rate is constant to within 30%. At these same sites, however the burial rate of He-3, determined using chronostratigraphic accumulation rates, varies by more than a factor of 3. The lowest burial rates, which occur north of the equator at 1 degreesN, 139 degreesW are lower than the global average rate of supply of extraterrestrial He-3 by 20% and indicate that sediment winnowing may have occurred. The highest burial rates, which are recorded at the equator and at 2 degreesS. are higher than the rate of supply of extraterrestrial He-3 by 100%, and these provide evidence for sediment focusing. By analyzing several proxies measured in core PC72 sediments spanning the past 450 kyr we demonstrate that periods of maximum burial rates of (230) Th, He-3, Be-10, Ti, and barite, with a maximum peak-to-trough amplitude of a factor of 6, take place systematically during glacial time. However, the ratio of any one proxy to another is constant to within 30% over the entire length of the records. Given that each proxy represents a different source (U-234 decay in seawater, interplanetary dust, upper atmosphere, continental dust, or upper ocean), our preferred interpretation for the covariation is that the climate-related changes in burial rates are driven by changes in sediment focusing.


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