Differences in Preferential Sorting of Fine Particles in the Panama Basin Over the Past 25 kyr: Effects on 230Th-derived Focusing Factors

In this study, we attempt to understand how preferential sorting of fine particles during redistribution processes in the Panama Basin affects the 230Th constant-flux proxy. Fine particles likely contain greater amounts of 230Th, so that preferential sorting of fine particles may bias sediment mass accumulation rates (MARs). We examined sediments that span the past 25 kyr from two new sediment cores retrieved within about 56 km of each other in the northern part of the basin (MV1013-01-'4JC', 5° 44.699'N 85° 45.498' W, 1730 m depth; MV1014-01-'8JC', 6° 14.038'N 86° 2.613' W, 1993 m depth). Core 4JC, closer to the ridge top that bounds the basin (Cocos Ridge), has a thin sediment drape, while the deeper core 8JC, has a thicker sediment drape and lies further from the ridge top. 230Th-derived focusing factors from 4JC are similar and suggest winnowing with average values of about 0.5 and 0.6 during the Holocene and the last glacial, respectively. For 8JC, calculated average focusing factors are significantly different and suggest focusing with values of about 2 during the Holocene and 4 during the last glacial. Since the two sites are close to each other, one would expect similar rain rates and, therefore, similar 230Th-derived MARs within similar windows of time, i.e., the rain rate should not vary significantly at each site temporally. In addition, the radiocarbon-derived sand (>63μm) MARs should behave similarly since coarser particles are likely not transported by bottom currents. Sand MARs are, indeed, similar during the Holocene and the last glacial at each site. During the last glacial, however, sand MARs are about a factor of 3 higher than those during the Holocene. On the other hand, there is little variability in the 230Th-derived MARs both spatially and temporally. We interpret the discrepancies between the radiocarbon-derived sand and 230Th-derived MARs as being due to preferential sorting of fine particles during the redistribution of sediments by deep-sea currents. The 230Th-derived focusing factors are being overestimated at the deeper site and vice versa at the shallower site, and the degree of inaccuracy varies temporally. We discuss this temporal variability and its relationship to deep-sea current velocities.