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Cosmogenic Silicon – 32 reveals extensive authigenic clay formation in deltaic systems and constrains the marine silica budget

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dc.contributor.advisor Aller, Robert C en_US
dc.contributor.advisor Cochran, James K en_US
dc.contributor.author Rahman, Shaily en_US
dc.contributor.other Department of Marine and Atmospheric Science en_US
dc.date.accessioned 2017-09-20T16:53:31Z
dc.date.available 2017-09-20T16:53:31Z
dc.date.issued 2016-12-01 en_US
dc.identifier.uri http://hdl.handle.net/11401/77762 en_US
dc.description 184 pg. en_US
dc.description.abstract Silica is an essential nutrient and an important control on primary production, the biological pump, and carbon cycling in the ocean. In our current understanding of the marine silica cycle, the known sources outweigh the sinks, by as much as 35%. The quantity of biogenic silica stored in lithogenic sediments along continental margins is poorly constrained. Cosmogenic 32Si (t1/2 ~ 140 yrs) was used in a novel way to constrain the quantity of reactive Si storage and early diagenetic reactions of Si in the highly mobile deltaic sediments along the coast of French Guiana, representative of deposits along the ~1600 km Amazon – Guianas coastline downdrift of the Amazon delta. A sequential leach was developed to extract and purify SiO2 from different operational pools in large samples of surface sediments (0 – 10cm). This methodology, a hot 1% Na2CO3 leach followed by a hot 4M NaOH leach, was adapted from the existing leaches widely used to estimate biogenic silica (bSi) content in marine sediments, and ultimately to constrain the global oceanic Si budget. 32Si activity was determined in each pool via its daughter product 32P. Results from several sites in coastal mudbanks near Kourou and Sinnamary indicate no detectable 32Si activity in the bSi fraction, whereas 32Si was detected in the Si-NaOH fraction after removal of bSi. The lack of detectable activity in the 1% Na2CO3 leach and its detection in the NaOH fraction (0.4 – 2.5 dpm) indicate that the method widely used to determine bSi content recovers only a minor fraction of the originally deposited reactive bSi in these deposits. The results are consistent with rapid alteration of biogenic silica and clay authigenesis or reverse weathering. They also demonstrate that the current estimate of biogenic silica storage in tropical deltaic sediments is significantly underestimated by at least one order of magnitude. Assuming an initial diatom specific activity range of ~5 – 40 dpm/kg SiO2, the 32Si activity in the NaOH fraction corresponds to a reactive Si storage of ~150 – 18,000 µmol Si/g sediment. This magnitude is more consistent with estimates of reactive Si (∑Sihr) storage in the Amazon delta based on modified operational leach techniques (a 0.1N HCl pretreatment step followed by extraction with 1% Na2CO3) that target poorly crystalline clays and with diagenetic modeling of pore water K+, F-, and Si(OH)4, though these modified leaches also appear to underestimate the amount of reactive Si stored along this system. To directly confirm whether these modified operational extractions underestimate reactive Si storage, a sequential extraction methodology was also developed to first isolate 32Si activity in the ∑Sihr fraction (0.1N HCl followed by 1% Na2CO3) and then extract any remaining 32Si from the residual fraction using 4M NaOH. 32Si activity was detected in both these fractions in sediments collected from two sites at Sinnamary, demonstrating that reactive Si (∑Sihr) storage was 2 – 3x what previous operational leaches indicated. A simple transport – advection model, assuming an activity of 15dpm/kg SiO2 in starting materials such as plankton, revealed a Si incorporation rate of 5 µmol Si/g/y along the Amazon – Guianas dispersal system and total Si burial of 0.3 Tmol Si/y. The combined Si storage in the Delta and along the coast to the Orinoco (0.4 Tmol/y) may account for 50% of the total dissolved Si delivery from the Amazon River to the global ocean. In addition to sites in the Amazon – Guianas mudbelts, sediment from 2 stations in the Gulf of Papua, Papua New Guinea, 1 station in the northern Gulf of Mexico near the Southwest Pass, and 1 station in Long Island Sound (Smithtown Bay) were also extracted for 32Si in the bSi fraction as well as the residual fraction after removal of bSi. Bulk 32Si activities in the residual fractions in the Gulf of Papua (0.5 – 0.7 dpm/kg sediment) were used to extrapolate Si storage in the outer topset and forset of the clinoform delta. Initial estimates of Si burial in this region, as indicated from 32Si activities, were ~0.10Tmol/y, or 74 – 99 % of the known total Si inputs to the Gulf. These results demonstrate that authigenic clay formation may be an important, and previously unnoticed/undetected?, sink of biogenic silica in this region. Unlike the Amazon – Guianas mudwave system and the Gulf of Papua deposits, 32Si activity was detected in the both the bSi (0.21 ± 0.04 dpm/kg sediment) and the residual fraction (0.44 ± 0.08 dpm/kg sediment) from the site in the Gulf of Mexico. A Si burial rate using the 32Si activity in the bSi fraction (assuming an activity of 15dpm/kg in starting Si materials) of 0.004Tmol/y was calculated over approximately 5000 km2 of the delta, whereas the burial rate calculated using the Si content in this same fraction from a classic bSi leach, was ~0.006Tmol/y. Adding the Si burial rate using the 32Si activity in the residual fraction (0.008Tmol/y) yielded a total storage per year of 0.012Tmol Si, ~10% of the total Si inputs (dissolved and amorphous Si) from the Mississippi – Atchafalaya river system. 32Si activity was also detected in the residual fraction (0.53 ± 0.08 dpm/kg sediment) after removal of ∑Sihr and using this activity yielded similar calculated rates of Si burial (~0.01 Tmol/y). In Smithtown Bay, Long Island Sound, 32Si activity was also detected in both the bSi (0.15 ± 0.05 dpm/kg sediment) and the residual (0.4 ± 0.2 dpm/kg sediment) fractions from the site in Smithtown Bay, Long Island Sound, yeilding a total Si storage estimate (assuming an activity of 15 dpm/kg in starting Si materials) of 1.6 x 10-3 Tmol/y over the entire Sound, comparable to estimates of Si storage calculated using the Si content in the classic bSi (1.1 x 10-3 Tmol/y) and the classic ∑Sihr (2.2 x 10-3 Tmol/y) leaches. It appears that reverse weathering is an important sink of Si in these deposits, and indeed may be in other temperate estuaries, and that classic bSi or ∑Sihr leaches can underestimate Si storage in these system by two to four - fold. In summary, authigenic clay formation is a significant sink of biogenic silica in tropical deltaic systems. Cosmogenic 32Si can be used to quantify the amount of reactive or biogenic Si buried in coastal sediment deposits more accurately than conventional operational leaches. Like tropical systems, initial findings suggest that reverse weathering plays a major role in Si burial in temperate estuaries and subtropical systems. en_US
dc.description.sponsorship This work is sponsored by the Stony Brook University Graduate School in compliance with the requirements for completion of degree. en_US
dc.format Monograph en_US
dc.format.medium Electronic Resource en_US
dc.language.iso en_US en_US
dc.publisher The Graduate School, Stony Brook University: Stony Brook, NY. en_US
dc.subject.lcsh Chemical oceanography -- Geochemistry en_US
dc.subject.other authigenic clay formation, reverse weathering, Silicon-32 en_US
dc.title Cosmogenic Silicon – 32 reveals extensive authigenic clay formation in deltaic systems and constrains the marine silica budget en_US
dc.type Dissertation en_US
dc.mimetype Application/PDF en_US
dc.contributor.committeemember DeMaster, David J en_US
dc.contributor.committeemember Froelich, Philip N en_US
dc.contributor.committeemember Zhu, Qingzhi en_US
dc.contributor.committeemember . en_US

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