Fig. 4 Pore water [Ge] as a function of [Si]. Lines connect points in order of increasing depth. Negative slope indicates Ge sink. MC18 is affected by irrigation. 50 60 70 80 90 100 120 140 160 180 [Ge], pM [Si], µM MC18 (400 m) 50 55 60 65 70 50 100 150 200 250 [Ge], pM [Si], µM MC15 (2 km) 20 40 60 80 100 20 40 60 80 100 [Ge] = 1.04*[Si] R 2 = 0.99458 [Si], µM [Ge], pM MC18 (400m) 20 30 40 50 20 30 40 50 [Ge] = 0.88*[Si] R 2 = 0.96514 [Si], µM [Ge], pM MC15 (2km) Fig. 3 Core incubation fluxes from sediment into bottom water, reflecting dissolving opal Ge/Si. 1. Surface water is enriched in Ge relative to Miss. Riv. probably due to diatoms discriminating against Ge, despite high surface [Si] (9 μM) (Fig 2). • Using salt balance and shelf water Ge/Si & [Si] (Fig. 6, top box), fractionation was calculated to be R opal /R water = 0.63, where R is Ge/Si ratio. 2. Different contribution of inputs (Fig. 1) is recorded by opal: • MC18 is more influenced by Mississippi river (higher Ge/Si in flux, Fig.3). • MC15 is more influenced by Atlantic water (lower Ge/Si in flux, Fig. 3). 3. As opal dissolves in the reducing shelf and slope sediments, Ge is removed into a non-opal sink, as indicated by: • Negative slopes of [Ge] vs [Si] plots (Fig. 4). • Ge/Si fluxed out of sediments is lower than Ge/Si in opal coming down (Fig. 4 & 6). • Porewater [Ge] and Ge/Si minima coincide with reduced Fe/Mn (Fig. 5). 4. Bio-irrigation may weaken non-opal Ge sink, by injecting bottom water into the sediment (Fig 4 & 5, MC18). An Investigation of Controls on Oceanic Ge/Si Ratios, a Potential Proxy for Changes in the Biogeochemical Cycling of Si Jotautas J. Baronas 1* , Douglas E. Hammond 1 , William M. Berelson 1 , James McManus 2 , Silke Severmann 3 1 University of Southern California 2 Oregon State University, 3 Rutgers University *Correspondence: baronas@usc.edu Introduction • Ge biogeochemical cycle is very similar to Si. • Both get removed from seawater by opal deposition but Ge is also sequestered in reducing shelf sediments (non-opal sink) 1 . • Ge/Si in seawater was lower during glacials , vs interglacials 2 , potentially caused by more effective Ge removal 3 . • The current global extent of this sink and the mechanism controlling its effectiveness is yet to be identified. • We measured Ge/Si in the Gulf of Mexico shelf water column, sediment pore waters and benthic fluxes to investigate Ge & Si cycling in a restricted, low-O 2 coastal environment. Results and discussion • Sediment cores were collected using multicorer during 2011 Jul/Aug cruise EN497 aboard R/V Endeavor. • Pore waters were sampled at near in-situ temperature using Rhizons. • Core incubations were done by stirring overlying water of sealed cores at in-situ temperature and taking series of samples over several days. • Water column samples were taken using CTD rosette. • [Si] was measured using colorimetric technique, analytical error <1.5%. • [Ge] was measured using Hydride-generation-ICP-MS, analytical error <3%. 0 5 10 15 20 25 30 35 Ge, pM Si, µM Mn 2+ , µM 0 50 100 150 200 Bottom water Sediment Concentration Sediment depth, cm 0.3 0.4 0.5 0.6 0.7 0.8 0.9 non‐opal Ge sink Ge/Si, pM/µM MC15 (2 km) 0 5 10 15 20 25 30 Ge, pM Si, µM Fe 2+ , µM 0 50 100 150 200 Bottom water Sediment Concentration Sediment depth, cm 0.4 0.5 0.6 0.7 0.8 0.9 1 non‐opal Ge sink Ge/Si, pM/µM irrigation MC18 (400 m) Fig. 5 Sediment pore water profiles of [Ge], [Si], [Fe 2+ ]/[Mn 2+ ], and Ge/Si, measured from composites of 5 cores at each site. References 1. Hammond D.E. et al., 2000, Geochim. et Cosmoch. Ac., 64 (14), 2453–2465 2. Shemesh A. et al., 1989, Paleocean., 4(3), 221-234 3. Hammond D. E. et al., 2004, Paleocean., 19, PA2016 4. Sutton J. et al., 2010, Glob. Biogeochem. Cyc., 24, GB2017 Summary and conclusions • GOM Ge/Si ratios are higher compared with global ocean average. • Extensive non-opal diagenetic Ge sink present in GOM shelf and slope sediments. • Local effects (biological fractionation, supply by different water masses) can have a big effect on Ge/Si delivered to the sediments. Fall Meeting 2012 American Geophysical Union Fig. 1 Map of sampling stations, and very qualitatively, the movement of main water masses in GOM: the Mississippi delta plume (yellow) and the Loop current (orange), each one carrying a distinct Ge/Si signal. Atlantic water Ge/Si = 0.75 Mississippi plume Ge/Si = 1.5 400m deep 2km deep Future work • Measuring solid phase opal (and non-opal?) Ge/Si in GOM sediments. • Constraining the effectiveness of non-opal Ge sink. • Using Ge isotopes to gain insight into fractionation processes. • Providing a better informed interpretation of paleo-Ge/Si record in opal. Fig. 6 Summary of Ge and Si cycling on GOM shelf. Numbers represent Ge/Si, pM/µM. *Calculated using salt balance for the surface box. **NADW value 4 used due to lack of data. All other values (except sedimentary opal & non‐opal) measured during cruise. Methods Station M (MC18) Station G (MC15) 500 1000 1500 2000 2500 0 20 40 60 80 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 5 10 15 20 25 30 Water depth, m Ge/Si ratio, pM/μM surface water deep water [Si], μM Fig. 2 Surface water [Si] drawn down, while Ge/Si enriched due to Mississippi input & biological fractionation Acknowledgments This research was funded by NSF grant 1061700. We would also like to thank the wonderful crew & science team of R/V Endeavor.