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

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. Assuming an initial diatom specific activity range of ∼5--40 dpm/kg SiO 2, the 32Si activity in the NaOH fraction corresponds to a reactive Si storage of ∼150--18,000 micromol Si/g sediment. This magnitude is more consistent with estimates of reactive Si (SigmaSi hr) storage in the Amazon delta based on modified operational leach techniques 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 SigmaSihr fraction (0.1N HCl followed by 1% Na 2CO3) and then extract any remaining 32Si from the residual fraction using 4M NaOH.;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.;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 SigmaSi hr 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, yielding 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 SigmaSihr (2.2 x 10-3 Tmol/y) leaches. It appears that reverse weathering is an important sink of Si in these deposits and that classic bSi or SigmaSi hr leaches can underestimate Si storage in these system by two to four-fold. (Abstract shortened by UMI.).