Browsing by Author "Wendt, Kathleen"

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    Biogeochemical patterns and processes in buoyant, deep-sea hydrothermal plumes
    (2011-08-11) Wendt, Kathleen
    Along the global mid-ocean ridge, sub-seafloor hydrothermal circulation results in the exchange of heat and chemical species between seawater and the ocean crust. The resulting thermally and geochemically altered fluids are vented at the seafloor. The mixing of cold, oxic deep-ocean waters with hydrothermal fluids creates plumes with physically and chemically dynamic features. Hydrothermal plumes represent a globally distributed interface where marine hydrothermal circulation exerts its biogeochemical influence on elemental budgets of ocean basins. The goal of the present study is to describe the microbiological niches created by physical and geochemical gradients in plumes. One of our central hypotheses is that microorganisms respond to and alter the geochemistry of hydrothermal plumes. To achieve this goal and test our hypothesis, a field study was undertaken at the Eastern Lau Spreading Center (ELSC). While multiple vent sites along the ELSC are included in the larger study, here we report on an integrated, biogeochemical investigation of a single buoyant plume within ABE vent field. A series of replicate sample sets were collected by in situ filtration at 0.5m, 40m, 200m within a buoyant plume using the ROV JASON. Above plume background and near bottom background sample sets were also collected. Hydrothermal plume particles in sample replicates or splits have been queried for bulk geochemistry, particle-by-particle mineralogy, and microbial community composition. These three data streams are being evaluated individually to characterize the geochemical and microbiological changes throughout the plume with respect to above and below plume backgrounds. In addition, an iterative and integrated analysis is being used to compare: (1) calculated mineralogy to direct measurements; and (2) predicted energy yields from chemoautotrophy to observed microbial composition.
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    Geochemical Profiles of Three Deep-sea Hydrothermal Plumes of the Eastern Lau Spreading Center
    (2013-08-09) Wendt, Kathleen;
    Inputs from deep-sea hydrothermal vents influence ocean geochemical budgets worldwide. Hydrothermal vents produce buoyant and chemically dynamic plumes that are rich in trace seawater elements. The mineral diversity in lower plumes is a direct result of interactions between local geology, hydrology, and biology. Because of this, differences in geochemical trends among naturally occurring plumes are detectable. Past studies have focused on geologic and bulk-chemistry differences between hydrothermal vents (Ferrini et al. 2008; Baker et al. 2005; Pearce et al. 1995; Tivey et al. 2005), while mineral differences among individual plumes remain poorly understood. This study examines and compares lower plume geochemistry of three hydrothermal vents located along the Eastern Lau Spreading Center (ELSC). Deep-sea hydrothermal plumes were sampled by in-situ filtration using ROV Jason2 and the SUPR Sampler (Breier et al. 2009). Samples were collected 0.5 meters above each vent onto polycarbonate membrane filters. Plume particles on the filters were examined by synchrotron radiation X-ray fluorescence (XRF) mapping. Subsequently, a total of 67 filter-bound particles underwent X-ray ray diffraction (XRD) to define the crystalline minerals. Differences among sites are compared and discussed regarding: (1) geologic setting; (2) vent fluid chemistry; and (3) lower-plume mineral species. Reduced metal polysulfides were detected in concentrations that coincide with previous bulk-chemistry studies (Mottl et al., 2011; Tivey et al., 2007; Ishibashi and Urabe, 1995). In addition, this study will introduce a new method to rate confidence levels associated with the identification of mineral species using X-ray microprobe XRD and XRF analysis.
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    Heinrich Stadials recorded in Northeast Brazilian Stalagmites
    (2015-05) Wendt, Kathleen
    During the last glaciation, periods of extreme cooling triggered massive freshwater and iceberg discharges into the North Atlantic. These periods of cooling, referred to as Heinrich Stadials, are believed to have caused an abrupt reorganization of atmospheric and oceanic circulation. Studying the behavior of tropical precipitation systems during Heinrich Stadials is important to understanding the response of tropical hydroclimate change to North Atlantic climate anomalies. In addition, tropical precipitation records may provide insight into the detailed timing and structure of Heinrich Stadials. Doing so is critical to understanding the underlying causes of Heinrich Stadials and their associated discharge events. The position of the Inter-Tropical Convergence Zone (ITCZ) plays a key role in the locality and amount of tropical precipitation worldwide. Paleo-records suggest that Heinrich Stadials triggered a southward migration of the ITCZ. The semiarid region of Northeast (NE) Brazil is located immediately south of the modern-day Atlantic ITCZ position. Short-lived speleothems that decorate the dry caves of NE Brazil suggest past periods of intense rainfall due to southerly migrations of the ITCZ. Previous studies have linked NE Brazil speleothem growth phases to Heinrich Stadials (Wang et al. 2004). Thus, reconstructing NE Brazilian pluvial periods will provide important insight into the chronology and structure of Atlantic ITCZ migrations associated with Heinrich Stadials. Here, we present a high-resolution, absolute dated, multi-stalagmite record of Heinrich Stadial (HS) 1, 4, 5 and 6 as recorded in NE Brazilian stalagmites. This thesis will focus on HS4 and HS1 time periods. Results show a detailed anti-correlation between NE Brazil and Northern Hemisphere subtropical records during HS4 and HS1, such as the Hulu Cave record from China (Wang et al. 2001). The synchronicity of these two distant records suggests a rapid transmission of atmospheric signals, likely through the global migration of the ITCZ. In addition, the 2-phased precipitation structure of HS1 and HS4 recorded in NE Brazil may reflect 2-stepped cooling observed in North and mid-Atlantic sea surface temperatures (Bond et al. 1992; Martrat et al. 2007). This supports the hypothesis that climate-ocean forcings were the underlying cause of Heinrich Events and highlights the relationship between mid-Atlantic sea surface temperatures and the position of the Atlantic ITCZ during the last glaciation. Arid NE Brazil is situated between the Amazon and the Atlantic rainforests. Evidence suggests that the pluvial periods associated with Heinrich Stadials promoted an ecological "bridge"� between both rainforests. This bridge may have permitted the transfer of species between rainforests. In this thesis, we compare NE Brazil speleothem precipitation records to NE Brazil palynological marine records to suggest the precise timing of rainforest expansion during HS1. Characterizing these NE Brazil pluvial anomalies are critical in understanding the link between climate change and the response of environmental systems.