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Excess Sediment Th: Transport Along the Sea Floor Or Enhanced Water

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Excess Sediment Th: Transport Along the Sea Floor Or Enhanced Water GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 22, GB1006, doi:10.1029/2007GB003057, 2008 Click Here for Full Article Excess sediment 230Th: Transport along the sea floor or enhanced water column scavenging? Wallace Broecker1 Received 9 July 2007; revised 27 August 2007; accepted 10 September 2007; published 5 February 2008. 230 [1] The sedimentary inventory of Th is often employed to distinguish between areas which receive excess sediment as the result of along-bottom transport from those which lose sediment as a result of winnowing. This process is referred to as ‘‘focusing.’’ A case is made here that, at least in the western equatorial Pacific, the excess 230Th in on-equator sediment is more likely scavenged from the overlying water column by the rain of organic matter than retransported along the bottom. If so, lateral variations in biological productivity may well contribute to the excess 230Th inventories elsewhere as well. Citation: Broecker, W. (2008), Excess sediment 230Th: Transport along the sea floor or enhanced water column scavenging?, Global Biogeochem. Cycles, 22, GB1006, doi:10.1029/2007GB003057. 1. Introduction is that the width of the excess 230Th-bearing zones on the two sides of the equatorial Pacific appears to mimic the [2] When combined with radiocarbon-based accumula- 230 zone of nutrient-rich surface water (see Figure 1). This zone tion rates, measurements of Th allow the accumulation is very broad adjacent to the Americas and extends further rate of this isotope in marine sediments to be calculated. If to the south of the equator than to the north. It progressively the resulting rate exceeds that for the production in the 230 narrows to the west and becomes more nearly centered on overlying water column, then the Th excess is attributed 230 the equator. At 140°W the zone of excess Th is not only to a process termed ‘‘focusing’’ [Francois et al., 1990, broader but, as is the high-nutrient content zone, is centered 2004; Lyle et al., 2005]. By focusing, most authors think to the south of the equator [Marcantonio et al., 2001]. The in terms of transport of reworked sediment across the sea enhanced abundance of nutrients and consequent high floor (i.e., it is akin to the formation of sand dunes or snow biological productivity is, of course, maintained by upwell- drifts). While marine sediment drifts are surely created in ing. This suggests to me that perhaps it is the high rain of this way, it is not clear whether this mechanism should be organic matter that is responsible for delivering the excess generalized to all sediments containing excess water column 230 230 230 Th to the underlying sediment. The Th excess depos- generated Th. It is possible that diffusion along isopycnal ited in the equatorial zone is supplied by lateral mixing of horizons in the presence of strong gradients in the rain of 230 equatorial waters with water adjacent to the equator. This organic matter enriches the Th in sediments beneath process is analogous to the removal of 231Pa by the high zones of enhanced productivity and correspondingly particle fluxes encountered near continental margins [Lao et depletes it in sediments beneath zones of lesser productivity. al., 1992] and in the Southern Ocean which deplete the [3] A case in point is the equatorial Pacific. Marcantonio water column in 231Pa thereby creating a gradient which et al. [2001] have shown that at 140°W the sediments in a allows it to be mixed in from the less depleted adjacent open zone extending several degrees north and south of the 231 230 230 ocean. The difference between Pa and Th is one of equator contain excess Th. These authors attribute this scale. As 231Pa has an order of magnitude longer water excess to the along-bottom lateral transport of reworked column residence time than 230Th, it is spread laterally sediment. By contrast, Higgins et al. [1999] have shown 230 230 thousands of miles while Th is spread only hundreds of that at 160°E the zone of excess Th is far narrower miles before being removed. extending only about 1° north and south of the equator. These authors suggest that, instead, the excess 230Th may be the result of scavenging from the water column. 2. Record at 160° East [4] Of course it is possible that both mechanisms are [5] One observation made in the western equatorial Pa- operative. Perhaps sediment reworking dominates in the cific must be explained by either focusing or scavenging. eastern equatorial Pacific, and scavenging dominates in the Broecker et al. [1999] show that at any given water depth no western equatorial Pacific. But I am not so sure. My reason difference in composition exists between off-equator and on-equator sediment. This similarity applies to both the ratio of CaCO to non-CaCO and the ratio of coarse CaCO to 1Lamont-Doherty Earth Observatory of Columbia University, Palisades, 3 3 3 New York, USA. fine CaCO3 (see Figure 2). Despite the strong decrease in CaCO3 with water depth, at any given depth these ratios are Copyright 2008 by the American Geophysical Union. no different in on- than in off-equator sediment. If along- 0886-6236/08/2007GB003057$12.00 GB1006 1of4 GB1006 BROECKER ET AL.: BRIEF REPORT GB1006 Figure 1. Mean annual concentration of nitrate in Pacific Ocean surface waters. Note that the equatorial maximum narrows and weakens to the west. Also, its southward skew gradually disappears to the west. Locations of the sediment traverses at 140°W and 160°E are shown. bottom sediment focusing were responsible for the excess rate of rain of debris produced by organisms living in the sedimentation on the equator, then, as stated by I. N. overlying surface water. McCave (personal communication, 2007): [7] As shown in Figure 3, for the on-equator cores at 160°E the rate of accumulation of non-CaCO3 material ‘‘The sand fraction (>63 microns) of deep sea sediments (mainly clay minerals) does not change significantly with (both foraminifera and quartz) is essentially immobile under water depth. Although the results for the off-equator cores all but the strongest flows. Focusing involves addition of have a greater scatter, the rate of accumulation of non-CaCO excess fine fraction (i.e., McCave and Hall, 2006, fig. 13). 3 The fact that the sand fraction does not vary as a function of material averages about one half that for on-equator cores. In latitude but only with depth suggests that the size distribu- my mind, this is more easily explained by scavenging than tion change is due only to selective dissolution and not by the along-bottom transport, for the latter would require focusing.’’ that the extent of sediment focusing for non-CaCO3 material be the same from 2.3 to 4.5 km water depth. [6] On the other hand, if water column scavenging is [8] One might ask then, why is there no trend with water responsible, then the rain rate of all entities reaching the sea depth in the accumulation rate of noncarbonate material? 230 floor (i.e., clay, CaCO3, and Th) must be governed by the Were this material uniformly distributed throughout the P Figure 2. CaCO3 content and size index, i.e., 100 Â (>63 mCaCO3/ CaCO3) for Ontong-Java Plateau core top samples as a function of water depth [Broecker and Clark, 2002]. Despite the fact that cores located more than 1° latitude north or south of the equator have accumulation rates approximately one half that for cores at the same water depth on the equator, their CaCO3 contents and size indices are not significantly different. 2of4 GB1006 BROECKER ET AL.: BRIEF REPORT GB1006 water column, then the scavenging hypothesis would pre- dict twice as high an accumulation rate at 4.5 km than at 2.3 km. Thus in order to explain the absence of a depth trend, it would have to be postulated that the noncarbonate material has a substantially higher concentration in the upper ocean than in the deep ocean. I. N. McCave (personal communication, 2007) states ‘‘In the central Pacific, the main source of terrigenous material is from wind- blown dust and thus has a maximum at the surface. It is then incorporated into aggregates and carried to the sea bed. So there is a good reason for the observation. There should be a small increase from the bottom nepheloid layer that might have acquired material by erosion of seamounts, but the concentration is so low that the flux must be very small.’’ 230 [9] However, as Th is produced uniformly throughout the water column, its sediment inventory should increase with water depth. Higgins et al. [1999] found that both on and off the equator the concentration of 230Th in the noncarbonate material was no higher in sediment from 4.4 km than in sediment from 3.2 km. However, as the expected difference is only about 30 percent, more 230Th analyses would be required to confirm the lack of a trend with water depth. [10] One important difference does exist between sedi- ments on and off the equator. It is the 14C age of the bulk CaCO3 [Broecker et al., 1999]. The ages of core top CaCO3 in the off-equator sediment are consistently about twice as Figure 3. Accumulation rates of CaCO3 and non-CaCO3 great as those for the on-equator sediment at the same water on the Ontong-Java Plateau as a function of water depth and depth (see Figure 4). This difference in 14C age is consistent latitude [Broecker et al., 1999]. As can be seen, the rate of with the observation that, at any given water depth, the non-CaCO3 accumulation, while twice as high on- than off- accumulation rate of sediment is twice as high on than off equator, is reasonably constant with water depth.
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