White Paper Title: 
The influence of biogeochemical processes on the exchange of material between the lithosphere and the oceans

Hydrothermal plume biogeochemical processes dictate the net effect of hydrothermal venting on the oceans just as estuarine biogeochemical processes dictate the net effect of river discharge. Previous hydrothermal studies have produced a largely abiotic conceptual model of hydrothermal plume formation (Mottl & McConachy, 1990; Rudnicki and Elderfield, 1993), and estimates of associated chemical fluxes (Kadko 1993; Elderfield and Schultz, 1996; German and Von Damm, 2000). However, there is also a body of evidence suggesting that hydrothermal plume chemistry is influenced by biotic processes as well. The first such indications were provided by Cowen et al. (1986), who reported microbially-catalyzed Fe/Mn oxidation-precipitation reactions plumes of the Juan de Fuca Ridge. Several other studies have since shown evidence of enhanced microbial CH4 and Mn oxidation in neutrally buoyant hydrothermal plumes (e.g. De Angelis et al., 1993; Mandernack and Tebo, 1993; O'Brien et al., 1998).

Within the last decade, a number of studies have provided new evidence for a coupling between abiotic and biotic plume processes. There has been new evidence of biomass production and microbial activity within plumes (Dick et al. 2009; Wakeham et al. 2001). Dynamics of specific microbial groups such as Mn- and ammonia-oxidizing microorganisms have been identified (Dick et al. 2009; Lam et al., 2004; Lam et al., 2008). There has been evidence that Mn-oxidizers can produce highly reactive biogenic Mn oxides (Dick et al. 2009). And recent studies also indicate that organic carbon binds a significant fraction of the dissolved and particulate metals in hydrothermal plumes by the processes of complexation (Sander et al. 2007; Bennett et al. 2008), and aggregation (Toner et al., 2009; Breier et al. submitted). In fact these studies suggest multiple mechanisms through which abiotic plume chemistry is coupled with biotic plume processes. These couplings are important from three perspectives: (1) hydrothermal energy sources stimulate microbial activity and productivity and potentially structure plume microbial communities; (2) microorganisms mediate the fate of elements and energy transferred from deep-sea hydrothermal vents into the water column; and (3) the aggregation of organic and inorganic material has a strong influence on the transport and fate of hydrothermal material and seawater scavenged trace elements.

This topic is timely, because recent studies suggest that processes active in hydrothermal plumes may significantly affect global ocean budgets. Specifically, it is now estimated that hydrothermal plumes may supply up to 25% of all deep-ocean dissolved Fe, which may also buffer the ocean Fe cycle on time-scales of 1000s yrs from short term (on the order of yrs) perturbations by processes such as aerosol deposition (Bennett et al. 2008; Tagliabue et al. 2010).

Issues of chemical transport and microbial utilization bear directly on those estimates. These issues, and the evidence and hypotheses associated with them, are ripe for synthesis and a synthesis paper could be developed to describe a coupled abiotic-biotic model of plume transport (see figure). Moreover, many of the underlying mechanistic abiotic-biotic couplings are not unique to the plume; but must also manifest themselves at and below the seafloor, and a complete understanding of chemical exchange between the lithosphere and the oceans must synthesize data and models across all of these zones. The biogeochemistry thematic working group will be a good forum for discussing these topics.

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Breier Fig. Graphical depiction of 6 general processes now thought to be coupled and influencing hydrothermal plume chemistry.