Sediment focusing in the Panama Basin, Eastern Equatorial Pacific Ocean
Highlights
► Downslope transport cannot explain high sediment focusing factors in Panama Basin. ► 230Th inventories in sediment may represent particle scavenging effects in basin. ► Detrital fluxes are highest during the Holocene in sediment within 300 km of margin.
Introduction
Essentially all sediments that reach the pelagic ocean floor are either derived from the continents through weathering processes or formed as a result of biological productivity in surface water. Contemporaneous climatic conditions largely affect the processes and mechanisms that bring these sediments to the ocean floor. For example, sediment intervals that record higher biogenic fluxes are often interpreted as being deposited at a time during which export production to the seafloor was increased. Similarly, intervals during which lithogenic particle fluxes are high can represent an intensified transport of continental material via rivers (if close to a continental margin) and/or wind. Thus, reconstruction of particle fluxes from oceanic sedimentary archives can broaden our understanding of past climate conditions.
Historically and to the present, sedimentary mass accumulation rates (MARs) have been estimated by multiplying the linear sedimentation rate (LSR), estimated using dated horizons (oxygen-isotope- or radiocarbon-derived), with sediment dry bulk density. This method measures the amount of sediment preserved at the sea floor but does not discriminate between vertically falling particles and those redistributed by a variety of horizontal advection processes. The xs230Th constant-flux proxy (CFP) method of determining mass accumulation rates is thought to “see through” such sediment redistribution processes and is purported to measure the true vertical flux (Bacon, 1984, Francois et al., 2004). The idea behind this constant-flux proxy lies in the different geochemical behavior of thorium and uranium in the oceanic water column. In seawater, 230Th is produced by the α-decay of 234U. Unlike uranium, which has a constant seawater concentration, thorium is extremely particle reactive, and the decay product, 230Th, is rapidly scavenged onto sinking particles so that the flux of 230Th to the ocean floor is identical to its rate of production in the water column (Bacon, 1984). Hence, MARs within an interval of sediment can be calculated by dividing the known production rate of 230Th by the concentration of 230Th within the same interval. Hence, the vertical flux of any component preserved in the sediment, Fi, can be calculated theoretically using the following equation:in which (conc)i is the concentration of component i; β is the production rate of 230Th in the water column (0.0267 dpm m− 3 yr− 1); Z is the water depth in m; and [xs230Tho] is the measured sedimentary 230Th activity corrected for decay, in situ production of 230Th from authigenic 234U, and detrital 230Th. The elegance of Eq. 1 is that the derived sedimentary flux is, by definition, solely the vertical component of the preserved sedimentary flux. Furthermore, one can solve for the ‘normalized’ sedimentary flux by measuring the concentration of xs230Th alone. The extent that such normalization works depends on how realistic the assumption is that the production of 230Th in the water column is equal to the flux of the scavenged 230Th to the underlying sediments (Bacon, 1984, Francois et al., 2004). If the postulated behavior of oceanic 230Th is correct, an added benefit to the constant-flux proxy methodology is that syndepositional sediment redistribution can be quantified by integrating the xs230Th inventory within an interval of sediment and comparing it to the integrated production of 230Th in the overlying water column over the time of accumulation (Suman and Bacon, 1989). Indeed, the ratio of these two parameters has been defined by a physical parameter called the “focusing factor, (Ψ)” (Suman and Bacon, 1989). A Ψ value of one implies that sediment has not been redistributed at the studied site. A Ψ value greater than one implies sediment in excess of what has been delivered vertically has been advected by deep-sea horizontal advection (i.e., focusing) to the studied site, while a Ψ value less than one implies winnowing or removal of sediment from the studied site at the time of sediment deposition (Francois et al., 2004). Model studies have shown that 70% of the ocean floor receives a 230Th flux within 30% of its production in the water column (Henderson and Anderson, 2003, Siddall et al., 2008), implying a sensitivity of the xs230Th profiling technique that is typically within +/−30%. This estimate must be considered somewhat tentative given the reliance of this result on the assumption that isopycnal and vertical diffusion is a reasonable approximation of ocean mixing processes inherent in many ocean models (Siddall et al., 2008).
Although age-model-derived and xs230Th-normalized MARs have been widely used in paleoceanographic research, in some cases the differently calculated MARs are significantly different, and, therefore, yield competing interpretations. Perhaps the best known of these discrepancies exists in the equatorial (west, central and east) Pacific Ocean (Broecker, 2008, Francois et al., 2007, Higgins et al., 1999, Kienast et al., 2007, Koutavas et al., 2002, Koutavas and Sachs, 2008, Kowsmann, 1973, Loubere et al., 2004, Lyle et al., 2005, Lyle et al., 2007, Marcantonio et al., 1996, Marcantonio et al., 2001a, Paytan et al., 1996, Thomas et al., 2000). Here, xs230Th-derived focusing factors, suggest that horizontal sediment transport almost always is higher (sometimes several times higher) than the vertical flux. The highest focusing factors (as high as 5.5; Kienast et al., 2007) are observed during the last glacial in the eastern equatorial Pacific (EEP) Ocean in the Panama Basin.
In the Panama Basin, using age-model-derived MARs, many investigators have concluded that particle fluxes during the last glacial were as much as 100% higher than those during the Holocene, and are caused by enhanced primary productivity (Lyle, 1988, Lyle et al., 2002, Paytan et al., 1996, Pedersen, 1983). However, xs230Th normalized MARs for sediments deposited during the last glacial suggest calcite fluxes that are 30–50% lower than those during the Holocene (Loubere et al., 2004). These authors contend that the higher glacial age-model-derived fluxes are due to sediment focusing processes in the Panama Basin. In addition, Kienast et al. (2007) reexamined several sites that were studied by others (Loubere et al., 2004, Lyle et al., 2005) in the Panama Basin and came to a similar conclusion; namely, that xs230Th-normalized MARs are lower and less variable than age-model-derived MARs, indicating varying degrees of sediment focusing.
Lyle et al. (2005) disagree with the interpretation that sediment focusing is widespread in the Panama Basin, and argue that xs230Th normalization overestimates the degree to which sediment redistribution processes are occurring in the EEP. They reason that the observed larger-than-expected inventories of sedimentary xs230Th in the EEP, that are in excess of those expected from a constant water column production rate of 230Th, can be attributed to increased boundary scavenging at the surface due to increased productivity close to the equator, in agreement with an analysis by Broecker (2008). However, within an efficient (low resolution) ocean circulation model, Siddall et al. (2008) found that particle scavenging effects are not sufficient to explain the additional xs230Th inventories measured in the Panama Basin. Using the Bern3D ocean model, they considered particle scavenging over a broad range of particle fluxes reaching up to 10 times higher than actual measurements in the equatorial Pacific region. Even at the highest end of this range of particle fluxes, the model by Siddall et al. (2008) suggests only a two-fold increase in the flux of 230Th over the production of 230Th in water column due to particle scavenging effects.
Kienast et al. (2007) propose that downslope transport of sediment from the east-west trending Carnegie Ridge, which forms the southern boundary of the Panama Basin, might explain the additional xs230Th in sediments of the Panama Basin. In this study, we test this downslope transport hypothesis by measuring xs230Th inventories of sediments deposited on the Cocos and Carnegie Ridges—regional topographic highs that surround the Panama Basin. In general, xs230Th inventories in sediment from the tops of ridges suggest sediment focusing factors that are greater than 1 for both the Holocene and glacial sediments. More importantly, sediment xs230Th inventories on the ridge tops are similar to those in the previously studied deeper cores (Kienast et al., 2007). If ridge tops were the source of extra xs230Th inventory in the basin, one would expect their focusing factors to be less than one and/or lower than those measured in the basin. We explore the potential causes for the larger-than-expected xs230Th inventories throughout the Panama Basin, including the effects of particle scavenging on 230Th fluxes to the seafloor.
Section snippets
Site selection and sampling strategies
We chose sites to test whether downslope transport from surrounding-ridge and within-basin topographic highs can explain the higher inventories of sedimentary xs230Th in the Panama Basin as suggested by Kienast et al. (2007). Cores were retrieved from the Carnegie and Cocos Ridges that ranged in depth from 712 m to 2230 m (Fig. 1; Table 1). We also selected two deeper cores (TR 163–22, just west of the Galapagos platform, and Y69-106P, just south of the Cocos Ridge) to add to the literature data
Results
Our MAR and xs230Th results in addition to those studied previously (Kienast et al., 2007) are presented (Table 1) with respect to their temporal and spatial (latitudinal, bathymetric and distance from continental margin) variability. In order to investigate temporal variability of MARs and sediment focusing factors, and to be consistent with the study of Kienast et al. (2007), we average such parameters for intervals of sediment deposited during the Holocene (0–13 ka; the Holocene) and the last
xs230Th from ridge tops surrounding Panama Basin
Previous 230Th studies suggest significant amounts of lateral redistribution of sediments (i.e., focusing factor values > 1) in the deeper sections (2700–3200 m) of the Panama Basin (Kienast et al., 2007, Kusch et al., 2010, Loubere et al., 2004). Similar xs230Th inventories with focusing factor values > 1 have been found throughout the equatorial Pacific Ocean suggesting that sediment focusing is a widespread phenomenon throughout the equatorial sector of the western (Higgins et al., 2002),
Summary and conclusions
In the Panama Basin, xs230Th-derived MARs are lower than age-model derived MARs, and lead to the prediction that significant sediment focusing (i.e., lateral redistribution of sediments by deep-sea currents) occurs. Downslope transport from surrounding ridge tops has been proposed as a source for excess inventory of xs230Th found in the deepest parts of the basin. We have tested this hypothesis and find a ubiquitous presence of larger-than-expected inventories of xs230Th on the tops and flanks
Acknowledgements
Sediment samples were provided by the core repositories at Lamont–Doherty Earth Observatory (supported by NSF grant OCE-07-51761), Oregon State University (supported by NSF grant OCE-0648164), and University of Rhode Island (supported by NSF grant OCE-06-44625). This research is funded by NSF grant OCE-0851056 to FM and ML. We thank Ken and Jane Williams for their generous support of the radiogenic geochemistry isotope facility at Texas A&M University. We thank Mark Siddall and two anonymous
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2021, Deep-Sea Research Part I: Oceanographic Research PapersCitation Excerpt :Despite a great proportion of the deep sea sediments in the Panama Basin corresponding to hemipelagic oozes, there is major terrigenous content along the American continental slopes and therefore a high heterogeneity in grain size (Lyle, 1992). Based on 232Th flux measurements on the basin, Singh et al. (2011) proposed that terrestrial detritus reach bottom sediments up to 300 km from the margin. Moreover, several biogenic facies have been proposed in shelf settings of the Panama Bight and their distributions and carbonate contents have been explained by terrigenous input (~45% originated from the San Miguel Bay) and the presence of insular bodies such as Coiba and Las Perlas (Reijmer et al., 2012).
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2016, Marine GeologyCitation Excerpt :Regionally, a simple relationship between sorting coefficients and focusing factors is not observed, possibly reflecting differences in the “upstream” properties of sediments delivered to the different sites and differences in local current regimes. Nevertheless, our study provides further evidence that the higher accumulation of 230Th is chiefly controlled by syndepositional lateral sediment transport in the EEP (Francois et al., 2004, 2007; Kienast et al., 2007; Kusch et al., 2010; Loubere et al., 2004; Marcantonio et al., 2001; Siddall et al., 2008), rather than by a higher scavenging efficiency in the equatorial Pacific (Broecker, 2008; Lyle et al., 2005, 2007; Singh et al., 2011). Furthermore, in light of the study by Kretschmer et al. (2010), our results suggest that the grain size bias on bulk 230Th concentrations in the EEP is small.