Phase heterogeneity in carbonate production by marine fish influences their roles in sediment generation and the inorganic carbon cycle

Michael A Salter,Chris T Perry,Rod W Wilson,Alastair R Harborne

doi:10.1038/s41598-017-00787-4

Abstract

Marine teleost fish are important carbonate producers in neritic and oceanic settings. However, the fates of the diverse carbonate phases (i.e., mineral and amorphous forms of CaCO3) they produce, and their roles in sediment production and marine inorganic carbon cycling, remain poorly understood. Here we quantify the carbonate phases produced by 22 Bahamian fish species and integrate these data with regional fish biomass data from The Bahamas to generate a novel platform-scale production model that resolves these phases. Overall carbonate phase proportions, ordered by decreasing phase stability, are: ~20% calcite, ~6% aragonite, ~60% high-Mg calcite, and ~14% amorphous carbonate. We predict that these phases undergo differing fates, with at least ~14% (amorphous carbonate) likely dissolving rapidly. Results further indicate that fisheries exploitation in The Bahamas has potentially reduced fish carbonate production by up to 58% in certain habitats, whilst also driving a deviation from natural phase proportions. These findings have evident implications for understanding sedimentary processes in shallow warm-water carbonate provinces. We further speculate that marked phase heterogeneity may be a hitherto unrecognised feature of fish carbonates across a wide range of neritic and oceanic settings, with potentially major implications for understanding their role in global marine inorganic carbon cycling.

Highlights

Marine bony fish have recently been identified as a globally important source of marine calcium carbonate[1]
Initial assumptions that fish carbonates are composed entirely of high Mg-calcite (HMC; 5–25 mol% MgCO3) and very high Mg-calcite (VHMC; >25 mol% MgCO3) formed the basis of a hypothesis wherein they strongly influence alkalinity–depth profiles in the ocean[1]. This is because the solubilities of these metastable carbonate phases exceed those of aragonite and calcite[17, 18], implying that their corresponding saturation horizons are located at shallower depths
A lack of quantitative data regarding the relative abundances of carbonate phases produced by different fish species and communities—both in neritic and oceanic settings—severely limits our capacity to understand the nature of these roles

Summary

Introduction

Marine bony fish have recently been identified as a globally important source of marine calcium carbonate[1]. The fate of most carbonates should either be accumulation as sediment if deposited above their respective saturation horizons, or dissolution in deeper ocean settings; surface water alkalinity remaining depleted in either case In this context, initial assumptions that fish carbonates are composed entirely of high Mg-calcite (HMC; 5–25 mol% MgCO3) and very high Mg-calcite (VHMC; >25 mol% MgCO3) formed the basis of a hypothesis wherein they strongly influence alkalinity–depth profiles in the ocean[1]. If established solubility relationships for these phases apply (i.e., in order of decreasing solubility: ACMC > VHMC > HMC > aragonite > LMC; solubility in calcites being positively correlated with MgCO3 content19, 23), fish-derived carbonate solubilities span two orders of magnitude, suggesting they will have markedly different post-excretion fates This implies that their roles in carbonate sediment cycling and inorganic carbon cycling in these neritic environments, and potentially in oceanic settings if phase heterogeneity applies universally, will strongly depend on what phases are being produced and in what ratios. The outcomes of this work are considered with respect to their implications for the fate and sedimentary significance of fish-derived carbonates in shallow-water regions of tropical and sub-tropical carbonate provinces, and their potential relevance to the inorganic carbon cycle in wider ocean settings

Methods

Results

Conclusion