The role of outcrop‐to‐outcrop fluid flow in off‐axis oceanic hydrothermal systems under abyssal sedimentation conditions

Abstract

[1] It has been proposed that ridge flank hydrothermal circulation by outcrop-to-outcrop (lateral) flow may be the dominant mode of oceanic hydrothermal circulation globally. In this model, the upper igneous crust is an aquifer overlain by low permeability sediments, and aquifer-ocean fluid exchange occurs through basement outcrops. Thermally induced pressure gradients drive fluid laterally from recharge outcrops to discharge outcrops. To test the global applicability of outcrop-to-outcrop flow, models of synthetic basement bathymetry representative of crust formed at different spreading rates, pelagic sediment supply and post depositional transport, and sediment hydraulic impedance are used to quantify the time-varying distribution of sediment and basement outcrops globally. Results suggest that basement outcrops may be 40-50% closer together than previously estimated. The modeled sediment and outcrop results are coupled with a two-dimensional model of outcrop-to-outcrop fluid flow and heat exchange in a vertically isothermal crustal aquifer to predict the spatial distributions of seafloor heat flow within the simulation region under this mode of hydrothermal circulation. It is found that both the time-varying average and standard deviation of modeled seafloor heat flow required by outcrop-to-outcrop flow simultaneously fit the global heat flow data if the average aquifer permeability decreases from $10 À9 m 2 to $10 À11 m 2 over the duration of the global heat flow deficit (to $65 Myr). This permeability range is consistent with other estimates of upper crustal permeability on comparable spatial scales, and supports the proposition that outcrop-to-outcrop fluid flow may be the dominant mode of off-axis hydrothermal circulation globally.

Alteration of ocean crust provides a strong temperature dependent feedback on the geological carbon cycle and is a primary driver of the Sr-isotopic composition of seawater

“…3). The average temperature of carbonate precipitation varies in a given crustal section, and between locations, due to spatial and temporal variations in the thickness of overlying sediment, hydrology of the oceanic crust and timing of carbonate precipitation (e.g., Anderson et al, 2013). The difference between the minimum and average temperature of carbonate precipitation records the average amount that carbonate saturated fluid is heated in the crust and is ∼9 • C (Fig.…”

“…This global dataset was filtered to exclude locations were the sedimentation rate was >20 m Myr −1 (compared to a global average of 3.5 m Myr −1 ; Anderson et al, 2012) because at such anomalously high sedimentation rates reactions within the sediment pile are likely to invalidate our assumption (see below) that the hydrothermal system is recharged by pristine seawater. Instead, a portion of the fluid recharging the hydrothermal system likely comes through the sediment and may be modified by reactions within the sedimentary pile.…”

“…This uncertainty reflects the possibility that fluid-rock reaction and carbonate precipitation may not have occurred at exactly the same temperature, as well as the uncertainty in the thermometer. Additionally, we assigned a 1σ uncertainty of 0.5 to the normalized time that the fluid was in the crust (t); this is based on models of the variation of flow path lengths in off-axis hydrothermal systems (Anderson et al, 2012). The seawater Sr-isotope curve was taken from a fit through the data of Veizer et al (1999).…”

“…We developed the simplest possible realistic model of the evolution of the hydrothermal fluid 87 Sr/ 86 Sr during fluid-rock reaction in an off-axis hydrothermal system. We assume a single box model in which bottom water enters the crust through an outcrop, flows through the crust reacting at some average temperature and precipitates secondary minerals, and discharges back into the ocean (Fisher and Becker, 2000;Anderson et al, 2012). Isotope exchange within the off-axis hydrothermal system is assumed to follow first-order kinetics (Lasaga, 1998) (2) where t = time; subscript SW = seawater; subscript hydro = the composition of the hydrothermal fluid and hence the carbonate minerals precipitated from it; and subscript basalt = fresh rock Srisotopic ratio, assumed to be 0.7025 for all modern oceanic sites and 0.7035 for the Troodos ophiolite.…”

“…Symbol colour reflects the temperature of carbonate precipitation using the carbonate O-isotope thermometer of Epstein et al (1953) and the age dependence of the O-isotopic composition of seawater of Coggon et al (2010). This is appropriate because the fluid δ 18 O is little modified by fluid-rock reaction within the crust due to the high waterto-rock ratio (Anderson et al, 2013). The same fractionation factor was used for both aragonite and calcite because for many carbonate O-and Sr-isotope analyses the mineralogy is unknown, or mixed, and using the same fractionation factor only translates into a few degrees Celsius uncertainty.…”