Barite Precipitation on Suspended Organic Matter in the Mesopelagic Zone

F Martinez-Ruiz,M T Gonzalez-MuñOz,P J Lam,A Paytan,F Jroundi,T J Horner,M M Abad,M Kastner

doi:10.3389/feart.2020.567714

Abstract

Mechanisms underlying barite precipitation in seawater and the precise depths of barite precipitation in the water column have been debated for decades. Here we present a detailed study of water column barite distribution in the mesopelagic zone at diverse stations in the open ocean by analyzing samples collected using multiple unit large volume in-situ filtration systems in the Pacific, Atlantic and Indian oceans. Our results demonstrate that barite is an organo-mineral particularly abundant at intermediate depths throughout the world’s ocean regardless of saturation state with respect to barite. This is confirming the notion of precipitation at depths of intense organic matter mineralization. Our observations further support the link between barite formation and microbial activity, demonstrated by the association of barite particles with organic matter aggregates and with extracellular polymeric substances (EPS). Evidence for microbial mediation is consistent with previous experimental work showing that in bacterial biofilms Ba binds to phosphate groups on cell surfaces and within EPS. This organo-accumulation promotes high concentrations of Ba leading to saturated microenvironments and nucleation sites favoring precipitation. The distribution of Ba isotopes in the water column and in particulate matter is also consistent with the proposed precipitation mechanism.

Highlights

Barium and barite are routinely used for reconstructing past export production in the ocean yet the processes linking barite formation to export production are still elusive
Barite shows typical rounded to oval morphologies ranging in size from nanometers to a few microns, and it is always associated with organic material, which in many cases has extracellular polymeric substances (EPS)-like morphology (Figures 2A,C,E)
Comparisons of suspended marine particulate matter obtained from diverse ocean sectors indicate barite formation at intermediate depths (200–600 m) in the mesopelagic zone independent of barite saturation state

Summary

Introduction

Barium and barite are routinely used for reconstructing past export production in the ocean yet the processes linking barite formation to export production are still elusive. Different algorithms have been suggested to correlate export production and excess Ba (total Ba concentration corrected for the lithogenic phase) or barite accumulation (e.g., Dymond et al, 1992; Francois et al, 1995; Paytan et al, 1996; Eagle et al, 2003), allowing the reconstruction of past ocean productivity (e.g., Dymond et al, 1992; Gingele and Dahmke, 1994; Nürnberg et al, 1997; Eagle et al, 2003; Ma et al, 2015). Quantification of export production from Ba proxies is still hindered by poor understanding of the mechanisms and processes leading to barite (the main phase carrying excess Ba) formation in the oceanic water column. Barite distribution in the oceanic water column is variable in space, time, and depth and such variability is not yet fully understood (e.g., Hernandez-Sanchez et al, 2011; Bates et al, 2017)

Results

Discussion

Conclusion