Sediment focusing in the central equatorial Pacific Ocean

Abstract

Abstract. At four sites in the central equatorial Pacific Ocean the flux of extraterrestrial 3He, determined using the excess 23øTh profiling method, is 8 x 10 -•3 cm • STP cm -2 ka -•. This supply rate is constant to within 30%. At these same sites, however, the burial rate of 3He, determined using chronostratigraphic accumulation rates, varies by more than a factor of 3.The lowest burial rates, which occur north of the equator at iøN, 139øW are lower than the global average rate of supply of extraterrestrial 3He by 20% and indicate that sediment winnowing may have occurred. The highest burial rates, which are recorded at the equator and at 2øS, are higher than the rate of supply of extraterrestrial 3He 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, 3He, løBe, 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 (234U 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.

Evaluating Cenozoic equatorial sediment deposition anomalies for potential paleoceanographic and Pacific plate motion applications

“…We therefore prefer a physical explanation for these features, in terms of bottom currents actively carrying particles to the equator and depositing them there by the equatorial focusing mechanisms described earlier (Marcantonio et al 2001;Mitchell and Huthnance 2013). These equatorial anomalies ( Figure 6) are of similar magnitude to the 230 Th--based focusing factors in Figure 2, with peak sediment thicknesses in some instances greater by a factor of two relative to those 1˚ away from the peak.…”

“…Although there is evidence in the modern seabed for sediment movements (Mitchell and Huthnance 2013) and movements have been interpreted from the 230 Th data (Marcantonio et al, 2001), vigorous equatorial signatures after the Brown--Purple seismic interval (after 8.55 Ma) are unclear in the seismic data, as the anomalies are not consistently developed. This is despite geophysical evidence for erosion in seabed data around the Clipperton Fracture zone (Johnson 1972) and farther south , and chirp sediment profiler data collected on AMAT03 cruise showing seabed truncations of reflectors.…”

“…A somewhat more peaked equatorial signature can be The MANOP--C equatorial long--term current meter data show a current varying in magnitude and direction over periods of 1--4 weeks or longer (Marcantonio et al 2001), though with a mean (residual) current in line with orientations of furrows on the modern seabed (Mitchell and Huthnance 2013). …”

“…Figure 3a (colored circles) shows Holocene ratios of sediment accumulation rate to overlying water--column production of 230 Th (Ψ) from data compiled by Kienast et al (2007) and McGee et al (2007). The high values in the central Pacific have been interpreted as caused by the movement of particles towards the equator by bottom currents, a process described as "sediment focusing" with Ψ the "focusing factor" (Marcantonio et al 2001). The exact cause of this effect is unclear, though Mitchell and Huthnance (2013) speculated that bottom Ekman currents, if they exist, would expand at the equator reducing shear stress (Apel, 1987) thus generally promoting deposition of particles.…”

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AbstractIf equatorial sediments form characteristic deposits around the equator, they may help to resolve the amount of northwards drift of the Pacific tectonic plate.Relevant to this issue, it has been shown that 230 Th has been accumulating on the equatorial seabed faster than its production from radioactive decay in the overlying water column during the Holocene (Marcantonio et al, 2001). Some researchers have argued that this reflects the deposition of particles with adsorbed 230 Th carried by bottom currents towards the equator ("focusing").

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