Coprecipitation and isotopic fractionation of boron in modern biogenic carbonates

The abundances and isotopic composition of boron in modern, biogenic calcareous skeletons from the Gulf of Elat, Israel, the Great Barrier Reef, Australia, and in deep-sea sediments have been examined by negative thermal-ionization mass spectrometry. The selected species (Foraminifera, Pteropoda, corals, Gastropoda, and Pelecypoda) yield large variations in boron concentration that range from 1 ppm in gastropod shells to 80 ppm in corals. The boron content of the biogenic skeletons is independent of mineralogical composition and is probably related to biological (vital) effects. The δ¹¹B values of the carbonates range from 14.2 to 32.2%. (relative to NBS SRM 951) and overlap with the δ¹¹B values of modern deep-sea carbonate sediments ( δ¹¹B = 8.9 to 26.2%.). The variations of δ¹¹B may be controlled by isotopic exchange of boron species in which ¹⁰B is preferentially partitioned into the tetrahedral species, and coprecipitation of different proportions of trigonal and tetrahedral species in the calcium carbonates. Carbonates with low δ¹¹B values (~ 15%.) may indicate preferential incorporation of tetrahedral species, whereas the higher δ¹¹B values (~30%.) may indicate (1) uptake of both boron species assuming equilibrium with seawater

(2) preferential incorporation of B(OH) ₄^- from in situ high-pH internal fluids of organisms that are isolated from seawater.

The B content and δ¹¹B values of deep-sea sediments, Foraminifera tests, and corals are used to estimate the global oceanic sink of elemental boron by calcium carbonate deposition. As a result of enrichment of B in corals, a substantially higher biogenic sink of 6.4 ± 0.9 × 10 ¹⁰ g/yr is calculated for carbonates. This is only slightly lower than the sink for desorbable B in marine sediments (10 × 10 ¹⁰ g/yr) and approximately half that of altered oceanic crust (14 × 10 ¹⁰ g/yr). Thus, carbonates are an important sink for B in the oceans being ~20% of the total sinks. The preferential incorporation of ¹⁰B into calcium carbonate results in oceanic ¹¹B-enrichment, estimated as 1.2 ± 0.3 × 10 ¹² per mil · g/yr. The boron-isotope composition of authigenic, well-preserved carbonate skeletons may provide a useful tool to record secular boron-isotope variations in seawater at various times in the geological record. The potential use of boron-isotope geochemistry in skeletons as a tracer for palaeoenvironments is demonstrated in Ostracoda and Foraminifera from the Gulf of Carpentaria, Australia. The δ¹¹B values of glacial-age, buried skeletons (4.0 and 4.9%., respectively) are lower than that of their modern equivalents (17.6 and 13.3%., respectively). This may reflect a "terrestrial" boron-isotope signature of the water in the gulf during the Late Quaternary when it was isolated from the ocean.