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The Role of Diatom Frustule Morphology in the B44A-0374 Formation of Reverse Weathering Products Rebecca A. Pickering and Jeffrey W. Krause Dauphin Island Sea Lab, Dauphin Island, AL University of South Alabama, Mobile, AL Introduction Analysis : TOC Analysis : HCl Leachable Reactive Silica ● The delivery of dissolved silicic acid (dSi) and burial of particulate biogenic silica (bSi) is Sample Weight % TOC dominated by processes occurring in land-sea interface systems. Total Organic Carbon (TOC) was 30 T. weiss - Plasma Ashed 3.99 ● Most methods to quantify bSi content of low latitude coastal sediments has underestimated the measured without vapor-phase T. weiss-Nitric Cleaned 0.91 amount of this material by failing to include authigenically altered bSi produced from reverse acidification to remove carbonate; Chaetoceros-Nitric Cleaned 0.76 25 weathering. samples will be acidified and run again Navicula- Nitric Cleaned 3.48 ● This process has been shown to be an important sink for Si and other elements but the to determine Inorganic C formation. <2 um Clay 1.27 To determine whether mechanisms regulating this process are still poorly understood, and it has been acknowledged 20 suspensions plus diatoms that the relatively rapid process of reverse weathering significantly influences elemental Si resulted in authigenic cycling and oceanic chemistry. Analysis : dSi 15 ● Goal: Here we report the preliminary results of ongoing laboratory batch experiments (5-15 alteration of bSi during the months), examining the formation of authigenic products using three morphologically distinct 5 month incubation, (umol/L) HCl BSi Leachable A time-series progression of T. weiss-Plasma Ashed A B C* D T.weiss-Plasma Ashed- NO SED A1 B1 C1* D1 diatom genera (Thalassiosira, Navicula, Chaetoceros) as a bSi matrix. samples were leached with 10 the samples (Table 2) are T. weiss-Nitric Cleaned E F G H* T. weiss-Nitric Cleaned-NO SED E1 F1 G1 H1* mild HCl following Krause seen in Figures 13-16. Chaetoceros-Nitric Cleaned I J K* L Chaetoceros-Nitric Cleaned- NO SED I1 J1 K1* L1 et al. (In Review). The Field Collections Navicula-Nitric Cleaned M* N O P Navicula-Nitric Cleaned- NO SED M1* N1 O1 P1 supernatant was run for 5 T.weiss Plasma Ashed silicic acid (Figure 17). Seawater and sediment 0 samples were collected from 1100 the 20 m isobath on the MS- 1000 AL coastal shelf in the 900 northern Gulf of Mexico Figure 17: HCl 800 Leachable Bsi after 5 (GoM, Figure 1). Month Suspensions 700 dSi (umol/L) dSi 600 Analysis : pH 500 A B C D A1 B1 C1 D1 9 400 Starting pH 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Ending pH Time (Days) 8.5 Figure 1: Field Locality of Seawater and Subsamples were Sediment Samples 8 Figure 13: Temporal changes in dSi in T.weissfloggi (Plasma Ashed) over five months. pH analyzed for pH before and after T.weiss Nitric Cleaned 7.5 Figure 2: Box Core Figure 3: Box Core Figure 4: Wire Mesh the five months Sediment Collection Sediment Grab Size Fractionation T.weiss PA T.weiss T.weiss PA PA T.weiss T. weiss N/S T. 7 N/SNavicula N/S N/S T. weiss N/S T. Navicula N/S Navicula Chaetoceros Chaetoceros Chaetoceros Chaetoceros No SED No SED < 2 µm No SED (Figure 18). 800 N/S - Sediments were collected with a box core (Figures 2 & 3) and Sed No - - - homogenized before using wire mesh sieves (Figure 4) onboard 700 to separate the <63 µm size fraction. Once back in the lab, Figure 18: Changes in Sample pH After 5 Months sediments were sieved down to <20 µm and <2 µm clay 600 suspensions were created through centrifugations (Figure 5). Figure 5: < 2 µm clay suspensions (umol/L) dSi Conclusions & Continuing Work 500 Methods E F G H E1 F1 G1 H1 ● Custom built chambers, rotation system, and HEPES buffer kept pH regulated 400 and successfully produced acid-leachable silica from diatom treatments with Bulk diatom frustules (Figure 6) were cleaned using nitric and sulfuric acids (Figure 7) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Time (Days) following protocols by Hasle (1978); additional Thalassiosira weissflogii was also stripped of sediment suspensions. organics using low-temperature plasma ashing. Cleaned material was measured for TOC and saved for later use (Figure 8). Figure 14: Temporal changes in dSi in T.weissfloggi (Nitric/Sulfuric cleaned) over five months. ● The acid-leachable silica produced over 5-months showed strong signal in the Chaetoceros Nitric Cleaned Navicula treatment, consistent with it having the most clear separation between the dSi replicates between treatments (i.e. with and without sediments). Thus, 600 Figure 7: the source bSi composition may be important. Cleaning 550 with Figure 8: 500 Figure 6: Nitric Cleaned ● Formation of these authigenic deposits in the diatom plus clay-suspension Original and Diatom 450 Diatom Sulfuric Frustule treatment has been observed to be rapid (i.e. 10 months) in previous studies; if 400 Material Acid Material dSi (umol/L) dSi the preliminary Navicula results are representative of field conditions, this 350 Cleaned frustule material was suspended in HEPES (15 mM)-buffered GoM seawater (0.2 µm timeline may be conservative. filtered) and placed in custom-built PVC chambers (Figure 9) that separated the diatoms (all 300 I J K L samples) and coastal clay suspensions (half of samples) by 12-14 kDa dialysis membranes 250 I1 J1 K1 L1 ● Ongoing work: Fluid chemistries will be monitored over the remaining 13 (Figure 10). The chambers were placed in a rotating wheel (2 rpm) to keep particles in 200 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 months of the experiment to characterize silicon isotope composition for the suspension (Figure 11). Time (Days) dSi. At the experiment conclusion, solid particle characterization will also be Figure 15: Temporal changes in dSi in Chaetoceros sp. (Nitric/Sulfuric cleaned) over five months. done (XRD, XRF, SEM and ICPMS to compare initial, time-point and final, and silicon isotope fractionation of authigenic products). Navicula Nitric Cleaned Figure 10: Figure 11: 12-14 kDa Rotating 1200 Dialysis Wheel to Keep Membranes 1100 Separating Particles in Acknowledgements Compartments Suspension 1000 Figure 9: Dual Compartment PVC Capsules University of South Alabama Department of Marine Sciences Graduate Fellowship and 900 the Dauphin Island Sea Lab for additional laboratory space and infrastructure access dSi (umol/L) dSi (courtesy of Dr. Kenneth Heck). We also thank; Liesl Cole, Alex Marquez, Sydney Chambers were filled with three different diatom genera and two 800 cleaning treatment types (Plasma ashed: Thalassiosira weissflogii, Acton, Eric Lachenmyer, Grant Lockridge, Caitlin Wessel, Emily Saad and Yantzee Hintz M N O P Nitric/Sulfuric cleaned: Thalassiosira weissflogii, Chaetoceros 700 for laboratory and logistical support. This work was funded through internal support at the M1 N1 O1 P1 Dauphin Island Sea Lab. calcitians, Navicula sp.). dSi was monitored periodically (Figure 600 Figure 12: 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Subsampling 12). After five months, 25% of the chambers were sacrificed and Time (Days) 100 µL analyzed; the experiment is ongoing with the remaining chambers. Questions? Aliquots. Figure 16: Temporal changes in dSi in Navicula sp. (Nitric/Sulfuric cleaned) over five months Contact the author at : [email protected] or (251) 861–2141 Ext: 2289.