Reply to comment by R Francois et al on Do geochemical

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PA1217 LYLE ET AL COMMENTARY PA1217, Figure 1 left Block model of the Panama Basin to estimate the 230Th inventory for the deep basin and. for the ridges and margins surrounding it The inventory is reported in Table 1 right Map of Panama. Basin from Lyle et al 2005 showing bathymetry and location of cores that have a depositional event at. the last glacial maximum, 5 Finally a wide spectrum of studies continues to find in the context of specific geological and sedimentological. that slowly accumulating sediments in deep basins through conditions. out the world s oceans consistently have smaller inventories 7 Figure 1 shows a map and a first order model of the. of 230Th than expected from seawater production a fraction Panama Basin divided up into sedimentary regimes to help. of which apparently is exported up and out of these basins clarify our arguments In the model we have divided up the. Dymond and Veeh 1975 Huh et al 1997 Moran et al basin and its surrounding boundaries into four types of basic. 2005 The deep basins of the ocean should behave as sinks sedimentary regimes 1 continental shelf and upper slope. not sources for 230Th if 230Th leaves the cycle as soon as it with an average depth of 500 m 2 shallow pelagic tops of. joins the sediment column More studies of 230Th from the aseismic ridges at an average depth of 1500 m 3 deep. deep gyre basins need to be made to understand the aseismic ridges with an average depth of 2000 m and 4 the. dynamics of 230Th in slowly accumulating sediments deep pelagic basin with an average depth of 3000 m We use. 0 5 cm kyr this model to explore quantitatively the production and. movement of 230Th and sediment within the Panama Basin. 2 A Mass Balance of the Panama Basin Note that the 230Th flux can be discussed independently of. total sediment flux, 6 Here we explore whether the assertion made by 8 The model maximizes the potential size of the source. Francois et al 2004 that the 230Th normalization method regions and the potential 230Th available for horizontal. can indeed correct burial fluxes sediment mass accumula transport into the basin Specifically we deliberately skewed. tion rates or MAR in the Panama Basin in the absence of the depth of the ridges and margins to the deep end of their. other information about the sedimentary environment We bathymetric range in order to maximize the 230Th flux that. reiterate that selective transport of the sediment fine fraction might be horizontally transported e g we allow for the. by horizontal advection contradicts their fundamental maximum possible excess 230Th to be available for redis. assumption that the 230Th ratio to bulk sediment rain at tribution Using the assumption that U is uniformly dis. any given site is constant and depends only on seawater tributed in the water column the vertical flux of 230Th to. U concentration and water depth We also show that near the seafloor is dependent only on the U concentration and. continental margins the large differences in water depth and water depth Francois et al 2004 The 230Th inventory is a. bulk sediment fluxes to different parts of the margin product of the vertical flux by the area for each regime. naturally set variable 230Th bulk sediment ratios and make Table 1. the correction based upon data from a single core imprac 9 Because the 230Th inventory is dependent upon water. ticable We also reiterate that 230Th should be treated as an depth shallow areas like the Middle America shelf and the. important tool in studies to understand sediment redistribu South American shelf make relatively small contributions to. tion but that it is not the whole toolbox and should be used the total 230Th inventory of the Panama Basin although. PA1217 LYLE ET AL COMMENTARY PA1217, Table 1 The 230Th Inventory for Panama Basin Based on Figure 1 12 The use of 230Th is further complicated when sedi. Volume ments fractionate by size During horizontal transport the. of Water fine sediment fraction always travels farther than the coarser. Water for 230Th Total 230Th fractions 230Th continuously adsorbs onto particles as they. Area Depth Production Inventory move through the water column and surface sediment It. m2 109 m m3 1012 dpm 1012, preferentially adsorbs on the fine fraction because adsorp.
Shallow blocks tion is correlated to surface area and the fine fraction has. Middle America shelf 136 500 68 1 8,South America Shelf 145 500 73 1 9. significantly higher surface area than coarser fractions Any. Cocos Ridge 222 2000 444 11 8 separation of fine fraction from coarse fraction along the. Malpelo Ridge 48 2000 96 2 6 transport route moves high amounts of 230Th but low bulk. E Carnegie Ridge 38 1500 57 1 5 mass of total sediment Removal of fines is often seen on. Carnegie Ridge 151 2000 302 8 1 ridge tops and is a feature of the ridges around the Panama. Galapagos Platform 30 1500 45 1 2, Shallow total 770 28 9 Basin Moore et al 1973 Size fractionation and transport. Deep Panama Basin 610 3000 1830 48 9 of the fines can raise the 230Th bulk sediment ratio by large. amounts and causes a huge overestimate of horizontal. sediment flux at the site of deposition The 230Th normal. they have a large impact on the sediment budget These two ization can thus either overestimate or underestimate the. shelves have a combined area equal to 46 of the deep mass of sediment transported horizontally It should not be. Panama Basin yet the combined 230Th inventory of the two used as a quantitative tool without other data to determine. shelves is less than 8 of the deep basin inventory The the source and transport process of the sediment. total 230Th inventory of the shallow rim of the Panama 13 Lyle et al 2005 explored two separate cases where. Basin plus the Malpelo Ridge in the center is less than the focusing factor was at least double the vertical partic. 60 of that of the Panama Basin floor Table 1 even ulate rain The excess sediment is assumed to come through. though the total shallow area is more than 25 greater than horizontal redistribution of the sediments from elsewhere. the basin floor In the first case Holocene and late Pleistocene sediments. 10 For 230Th flux corrections to work the 230Th activity under the Pacific equator from 86 W to 161 E Higgins et. must be constant as Francois et al 2004 pointed out al 1999 Marcantonio et al 2001 Loubere et al 2004 all. Imagine for example that pure 230Th particles are advected typically have a focusing factor near 2 meaning that the. to a point where all the vertical sediment flux is preserved model predicts that half the sediment was derived from. and buried in the sediment column The focusing factor will elsewhere However in no case has a source region been. then be greater than 1 because the 230Th burial will be identified for the excess sediment nor is there even a. greater than the water column production The normaliza plausible delivery method In the Panama Basin where. tion will overcorrect for the horizontally advected sediment the high surrounding ridges provide a possible source. because 230Th is being added without any additional sedi region the few published data also reveal a focusing factor. ment The same situation will occur at any time the 230Th of 2. activity of the horizontally advected sediment is substan 14 There is also a mass accumulation rate MAR event. tially higher than that of the vertical particulate rain The at the last glacial maximum in the eastern Pacific including. converse is also true Horizontally advected sediments with most of the Panama Basin The 18 ka MAR event has been. lower 230Th activity than that of the vertical particulate rain attributed to high productivity Pedersen 1983 Lyle et al. will produce an undercorrection of the focused flux 1988 Pedersen et al 1991 Lyle et al 2002 and is found. 11 Ocean margins are areas of high particulate rain but throughout much of the Panama Basin on top of the. low 230Th inventory Erosion from the shelves should Carnegie Ridge and in the northern Peru Basin Lyle et. contribute large amounts of sediment but low amounts of al 2005 Loubere et al 2004 and Francois et al 2004. Th i e a low 230Th bulk sediment ratio In contrast contend through 230Th normalization that the vertical rain of. erosion and redistribution within the deep basin will have a sediment did not change during the 18 ka MAR event. much higher 230Th bulk sediment ratio For example if Instead they propose that sediment focusing increased from. particulate rain to the shelf zones is 5 times higher than the 2 to a factor of 8 In other words Loubere et al 2004 and. vertical component of particulate rain to the pelagic basins Francois et al 2004 contend that there was no produc. then the 230Th bulk sediment of the shelf environment is 30 times tivity event at 18 ka but instead a major sediment transport. smaller than that of the pelagic basin The difference in event. water depth between the basin and the shelf causes the water 15 Lyle et al 2005 determined that local redistribution. column production of 230Th to be 6 times higher in the basin on a scale of tens of kilometers has caused depositional. than over the shelf Thus horizontal movement of 230Th variation in the range of 30 to 50 of the total sediment flux. derived from basin sediments will mark a movement of within the abyssal hill topography of the western Panama. 30 times less sediment than the same amount of 230Th Basin This is the case equivalent to what Francois et al. derived from the shelf One needs to know the various 2004 referred to as bottom nepheloid transport We also. sources of sediment that accumulate at any given site their noted that adjacent depositional areas maintained that level. relative contribution to the total sediment and their original of difference in burial for over 2 million years even during. Th bulk sediment ratio in order to convolute the original the 18 ka MAR event Time series of MAR at different. sediment rain rate from the 230Th signature,PA1217 LYLE ET AL COMMENTARY PA1217. spots in the basin when normalized for different average sediment movements as large as possible This depositional. MAR have coherent changes in deposition region shown as sink region on Figure 1 has an area of. 16 These observations imply that there is a small scale 46 109 m2 and has a vertical 230Th inventory of 3 2. 10 km scale syndepositional focusing that affects the 1012 dpm using an average water depth of 2500 m. average rate of burial but not the time series the changes in 20 As Table 1 shows there is sufficient area on the. MAR through time Proposed high focusing events like Carnegie Ridge to provide the excess 230Th inventory to. the 18 ka MAR event must be derived from outside the the sink region in the Holocene The 230Th inventory of the. local near bottom region to leave the coherent MAR time Carnegie Ridge is 2 5 times as large as the sink zone so. series The additional sediment must appear either through a the Carnegie ridge can produce a focusing factor of 3 5 if. change in vertical particle flux e g export production or the entire 230Th inventory is stripped from the Carnegie. by high transport from a distal sediment source The only Ridge and delivered to the sink zone However one result of. likely source for horizontally advected sediment to the deep this scenario is that the Carnegie Ridge could not supply. Panama Basin is the high topography surrounding it large amounts of 230Th or sediment to the rest of the. 17 Moore et al 1973 have shown that the ridges Panama Basin. surrounding Panama Basin provide additional sediment to 21 Because deep water moves into the Panama Basin. the basin and that much of the horizontally advected from the south and must be exported over the ridge tops. sediment may derive from water inflow into the Panama any water and any horizontally advected sediment on Cocos. Basin across a low saddle in the Carnegie Ridge see Ridge the northern boundary of the Panama Basin moves. Figure 1 Tsuchiya and Talley 1998 have shown that the out and away from the Panama Basin The Cocos Ridge. density salinity and temperature of western Panama Basin should thus supply minimal amounts of horizontally. deep waters are consistent with flow into the region from advected sediment to the deep Panama Basin. the south over the Carnegie Ridge Lonsdale and Malfait 22 If the Carnegie Ridge is supplying sediment to the. 1974 observed sand waves in the saddle of the Carnegie sink area to achieve a focusing factor of 2 the rest of the. Ridge at a depth of 2650 m about the same depth as core basin receives sediment and 230Th from the Carnegie Ridge. Y69 71 featured by Lyle et al 2005 Francois et al to achieve a maximum focusing factor of about 1 1 assum. 2004 and Loubere et al 2004 Y69 71 is located over ing all sediment is removed from the Carnegie Ridge which. 100 km to the northwest of the Carnegie Ridge saddle it is not Therefore we suggest that the best place to look. however Figure 1 The Carnegie Ridge is clearly a viable for sediment focusing around Y69 71 is to examine sedi. source for horizontally advected sediment However can it ments elsewhere in the Panama Basin and do the mass. supply enough sediment or 230Th to match the model based balance. sediment focusing It is important to explore this question 23 At the LGM there is a major mass balance problem. because the 18 ka MAR event is also found on the because a focusing factor of 8 was measured in the sink. shallowest part of the Carnegie Ridge V19 27 Lyle et region for an added 230Th inventory of 22 4 1012 dpm. Further explorations of 230Th normalization Mitchell Lyle 1 Nicklas Pisias 2 Adina Paytan 3 Jose Ignacio Martinez 4 and Alan Mix3 Received 29 September 2006 revised 9 December 2006 accepted 2 January 2007 published 6 March 2007 Citation Lyle M N Pisias A Paytan J I Martinez and A Mix 2007 Reply to comment by R Francois et al on Do geochemical estimates of sediment

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