Hydrothermal impacts on trace element and isotope ocean biogeochemistry

Abstract

Hydrothermal activity occurs in all ocean basins, releasing high concentrations of key trace elements and isotopes (TEIs) into the oceans. Importantly, the calculated rate of entrainment of the entire ocean volume through turbulently mixing buoyant hydrothermal plumes is so vigorous as to be comparable to that of deep-ocean thermohaline circulation. Consequently, biogeochemical processes active within deep-ocean hydrothermal plumes have long been known to have the potential to impact global-scale biogeochemical cycles. More recently, new results from GEOTRACES have revealed that plumes rich in dissolved Fe, an important micronutrient that is limiting to productivity in some areas, are widespread above mid-ocean ridges and extend out into the deep-ocean interior. While Fe is only one element among the full suite of TEIs of interest to GEOTRACES, these preliminary results are important because they illustrate how inputs from seafloor venting might impact the global biogeochemical budgets of many other TEIs. To determine the global impact of seafloor venting, however, requires two key questions to be addressed: (i) What processes are active close to vent sites that regulate the initial high-temperature hydrothermal fluxes for the full suite of TEIs that are dispersed through non-buoyant hydrothermal plumes? (ii) How do those processes vary, globally, in response to changing geologic settings at the seafloor and/or the geochemistry of the overlying ocean water? In this paper, we review key findings from recent work in this realm, highlight a series of key hypotheses arising from that research and propose a series of new GEOTRACES modelling, section and process studies that could be implemented, nationally and internationally, to address these issues.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

Keywords: GEOTRACES; hydrothermal activity; ocean biogeochemistry; trace elements and isotopes.

Figure 1.

The distribution of dissolved Fe in the Atlantic Ocean based on data from the GEOTRACES Intermediate Data Product [15]. Note the presence of Fe-rich plumes centred upon, but dispersing away from the Mid-Atlantic Ridge axis.

Figure 2.

Cross-sections of (a) dissolved Fe and (b) excess dissolved ³He in the hydrothermal plume dispersing west from the East Pacific Rise near 15° S (data from [24]).

Figure 3.

Illustration (a) and simplified box model (b) of key processes identified by SCOR-InterRidge WG 135 as critical to understanding the impact of hydrothermal venting on the deep ocean Fe and organic carbon cycle (redrawn from [35]). (Online version in colour.)

Figure 4.

Map, modified from [58], of known hydrothermal fields along the global ridge-crest as inferred from hydrothermal plume signals (circles) or from direct seafloor observations (squares). Sites located prior to 2000 are colour-coded blue while more recent discoveries are coloured red. Note that the majority of discoveries made worldwide in the period 2000–2013 were all located in the southwestern Pacific Ocean (red and white ellipse) in a region extending from the northern Lau Basin to the North Island of New Zealand.

Figure 5.

Map of all active high-temperature vent sites that have been confirmed from direct seafloor observations, to date, along the slow spreading northern Mid-Atlantic Ridge [6]. Venting extends from the Niebelungen vent site near Ascension Island, just south of the Equator to the Moytirra vent site, just north of the Azores. Colour coding differentiates between high-temperature sites of hydrothermal venting that are either in a magmatic setting (red) or a tectonic setting (yellow). Note that vent sites located in tectonic settings may be either basaltic-hosted (e.g. TAG) or ultramafic-influenced (e.g. Rainbow).