Methane, Manganese, and Helium in Hydrothermal Plumes following Volcanic Eruptions on the East Pacific Rise near 9°500N

As part of a rapid response cruise in May 2006, we surveyed water column hydrothermal plumes and bottom conditions on the East Pacific Rise between 9°46.0\u27N and 9°57.6\u27N, where recent seafloor volcanic activity was suspected. Real-time measurements included temperature, light transmission, and salinity. Samples of the plume waters were analyzed for methane, manganese, helium concentrations, and the δ13C of methane. These data allow us to examine the effects of the 2005–2006 volcanic eruption(s) on plume chemistry. Methane and manganese are sensitive tracers of hydrothermal plumes, and both were present in high concentrations. Methane reached 347 nM in upper plume samples (250 m above seafloor) and exceeded 1085 nM in a near-bottom sample. Mn reached 54 nM in the upper plume and 98 nM in near-bottom samples. The concentrations of methane and Mn were higher than measurements made after a volcanic eruption in the same area in 1991, but the ratio of CH4/Mn, at 6.7, is slightly lower, though still well above the ratios measured in chronic plumes. High concentrations of methane in near-bottom samples were associated with areas of microbial mats and diffuse venting documented in seafloor imagery. The isotopic composition of the methane carbon shows evidence of active microbial oxidation; however, neither the fractionation factor nor the source of the eruption-associated methane can be determined with any certainty. Considerable scatter in the isotopic data is due to diverse sources for the methane as well as fractionation as methane is consumed. One sample at +21% versus Peedee belemnite standard is among the most enriched methane carbon values reported in a hydrothermal plume to date

Trace metals in the central California Current upwelling system

The objective of this dissertation was to develop and utilize a multi-element analysis method to determine sources and distributions for a suite of trace metals in the central California Current upwelling regime (cCCS). Chapter 1 involved the development of this multi-element method and its use in the U.S. GEOTRACES inter-calibration efforts. Chapters 2 and 3 investigated sources of Fe and its importance as a bottom up control on phytoplankton growth of the cCCS. Coastal surface Fe concentrations were related to continental shelf width and upwelling strength, and benthic boundary layer Fe concentrations were high in regions with a wide shelf and/or very low oxygen concentrations. Several regions with narrow continental shelves (Big Sur Coast and Pt. Arena to Cape Mendocino) demonstrated evidence for Fe limitation of diatom blooms (Chapter 2). The dominant Fe source to the offshore transition zone (TZ) is from the transport of coastally upwelled waters offshore via filaments, though Fe is rapidly drawn down as these waters move offshore. The TZ thus exhibits residual NO3- concentrations (5-15 μg L-1), very low Fe (<0.2 nmol kg-1), and relatively low and constant chlorophyll concentrations (1-2 μg L-1). Additional Fe delivery via offshore wind curl induced upwelling and/or vertical mixing is not sufficient to accompany NO3- delivered to the surface. Thus, the TZ is a broad region of the cCCS exhibiting evidence for Fe limiting conditions (Chapter 3). Chapter 4 presented seasonal sources and distributions for a suite of trace metals (Mn, Fe, Co, Ni, Cu, Zn, and Cd) relative to macronutrients in the cCCS. Upwelling sources of Ni, Zn, and Cd were from the ocean interior and internal biogeochemical cycling. Conversely, Mn, Fe, Co, and Cu had an external source to upwelling waters in the continental shelf sediments. There is an increased upwelling source of Co and Mn later in the summer as shelf sediments become highly reducing. Surface Fe, Zn, Cd, and Co showed evidence for preferential drawdown relative to NO3- indicating a changing metal to carbon assimilation ratio in the environment

Distinct Pools of Dissolved Iron‐binding Ligands in the Surface and Benthic Boundary Layer of the California Current

Organic dissolved iron (dFe)—binding ligands were measured by competitive ligand exchange—adsorptive cathodic stripping voltammetry (CLE‐ACSV) at multiple analytical windows (side reaction coefficient of salicylaldoxime, αFe(SA)2 = 30, 60 and 100) in surface and benthic boundary layer (BBL) samples along the central California coast during spring and summer. The weakest ligands were detected in the BBL at the lowest analytical window with average log KcondFeL,Fe\u27 = 10.2 ± 0.4 in the summer and 10.8 ± 0.2 in the spring. Between 3% and 18% of the dFe complexation in the BBL was accounted for by HS, which were measured separately in samples by ACSV and may indicate a source of dFe‐binding ligands from San Francisco Bay. The strongest ligands were found in nearshore spring surface waters at the highest analytical window with average log KcondFeL,Fe\u27 = 11.9 ± 0.3, and the concentrations of these ligands declined rapidly offshore. The ligand pools in the surface and BBL waters were distinct from each other based on principal components analysis, with variances in the BBL ligand pool explained by sample location, and variance in surface waters explained by water mass. The use of multiple analytical window analysis elucidated several distinct iron‐binding ligand pools, each with unique distributions in the central California Current system

Iron-binding Ligands and Humic Substances in the San Francisco Bay Estuary and Estuarine-influenced Shelf Regions of Coastal California

Dissolved iron (dFe) and organic dFe-binding ligands were determined in San Francisco Bay, California by competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-ACSV) along a salinity gradient from the freshwater endmember of the Sacramento River (salinity \u3c 2) to the mouth of the estuary (salinity \u3e 26). A range of dFe-binding ligand classes was simultaneously determined using multiple analytical window analysis, involving titrations with multiple concentrations of the added ligand, salicylaldoxime. The highest dFe and ligand concentrations were determined in the low salinity end of the estuary, with dFe equal to 131.5 nmol L− 1 and strong ligand (log KFeL,Fe′cond role= presentation \u3e ≥ 12.0) concentrations equal to 139.5 nmol L− 1. The weakest ligands (log KFeL,Fe′cond role= presentation \u3e \u3c 10.0) were always in excess of dFe in low salinity waters, but were rapidly flocculated within the estuary and were not detected at salinities greater than 7. The strongest ligands (log KFeL,Fe′cond role= presentation \u3e \u3e 11.0) were tightly coupled to dFe throughout the estuary, with average excess ligand concentrations ([L]–[dFe]) equal to 0.5 nmol L− 1. Humic-like substances analyzed via both CLE-ACSV and proton nuclear magnetic resonance in several samples were found to be a significant portion of the dFe-binding ligand pool in San Francisco Bay, with concentrations ranging from 559.5 μg L− 1 to 67.5 μg L− 1 in the lowest and highest salinity samples, respectively. DFe-binding ligands and humic-like substances were also found in benthic boundary layer samples taken from the shelf near the mouths of San Francisco Bay and Eel River, suggesting estuaries are an important source of dFe-binding ligands to California coastal shelf waters

Iron-binding ligands and humic substances in the San Francisco Bay estuary and estuarine-influenced shelf regions of coastal California

Dissolved iron (dFe) and organic dFe-binding ligands were determined in San Francisco Bay, California by competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-ACSV) along a salinity gradient from the freshwater endmember of the Sacramento River (salinity b2) to the mouth of the estuary (salinity N26). A range of dFe-binding ligand classes was simultaneously determined using multiple analytical window analysis, involving titrations with multiple concentrations of the added ligand, salicylaldoxime. The highest dFe and ligand concentrations were determined in the low salinity end of the estuary, with dFe equal to 131.5 nmol L −1 and strong ligand (log K . Humic-like substances analyzed via both CLE-ACSV and proton nuclear magnetic resonance in several samples were found to be a significant portion of the dFe-binding ligand pool in San Francisco Bay, with concentrations ranging from 559.5 μg L −1 to 67.5 μg L −1 in the lowest and highest salinity samples, respectively. DFe-binding ligands and humic-like substances were also found in benthic boundary layer samples taken from the shelf near the mouths of San Francisco Bay and Eel River, suggesting estuaries are an important source of dFe-binding ligands to California coastal shelf waters