Halogens dissolved in interstitial water reveal the origin of migrating fluids in sediments of the Alboran Sea (western Mediterranean)

Fluid migration in sedimentary basins has profound effects on a range of geological processes, including the methane cycle, tectonic and sedimentary geohazards, and microbial communities in the oceans. The Alboran Sea is a tectonically active basin characterized by contourite drifts that host migrating fluids, expressed in places by pockmarks and mud volcanoes, the latter associated with seafloor methane seepage. In this study, we examine the composition and origin of near-seafloor fluids in the Alboran Sea using sediment cores (up to 20 m long) from a pockmark field (site CL06), a nearby background area (site CL04) and a fault zone (site CL55).

We use halogens (Cl, Br, and I) dissolved in interstitial water to understand the origin of fluids in the Alboran Sea. Chlorine is considered a conservative ion in interstitial water geochemistry, its concentration changing with pore water salinity. Iodine has a strong biophilic character and is incorporated in organic matter deposited with sediments, which during burial decomposes in response to geothermal heat or microbial activity to produce methane. Iodine and methane concentrations are strongly correlated and highly concentrated compared to seawater, so that iodine has been used as a methane tracer. Bromide also has a weak biophilic character and behaves similarly to iodine.

Interstitial water was extracted aboard ship using Rhizon samplers. Chloride concentration was determined by ion chromatography (ICS-1600, DIONEX) at the Tokyo University of Marine Science and Technology; iodine and bromine concentrations were determined by Inductively coupled plasma mass spectrometry (ICP-MS Agilent 7500) at Micro Analysis Laboratory, Tandem accelerator (MALT), University of Tokyo.

The results reveal halogen profiles that differ between the pockmark and fault sites, providing evidence of different modes of fluid migration within the contourite drifts of the Alboran Sea:

(1)Pockmark and background sites: surprisingly, halogen profiles are similar at these two sites. Cl concentration decreases with depth from 610 to 590 mM over the 15 m length of the cores, a trend indicating fresher water is present in deeper sediments. I and Br concentrations increase with depth (I: 0 to 70 µM, Br: 760 to 820 µM). I and Br are strongly enriched (up to 8% and 60%, respectively) by a deep fluid source, which may relate to high TOC or evaporated seawater in deeper sediment.

(2)Fault zone site: in contrast to the other two sites, Cl concentration increases with depth from 600 to 610 mM over the 16 m length of the core 55, a trend indicating saline water is dominant in deeper sediments. I and Br concentrations increase with depth (I: 35 to 70 µM, Br: 800 to 830 µM). I and Br concentrations in near-seafloor sediments are usually less strongly affected by organic decomposition, with concentrations as low as seawater; however, at site 55, I and Br are strongly enriched in near-seafloor sediments. This observation suggests vertical fluid migration is active and reaches the seafloor to maintain high I and Br concentrations.