Calcareous algae bioclast contribution to sediment enrichment by arsenic on the Brazilian subtropical coast

Abstract

Arsenic levels (up to 130 mg kg−1) substantially exceeding the official threshold have recently been documented in beach and nearshore sediments along more than 50 km of coastline in the Brazilian state of Espírito Santo between 19°50′ and 20°12′S. In an attempt to assess the sources of this enrichment, we performed a study on arsenic distribution in the main mineral substances and living organisms in the beach environment. Laboratory tests on arsenic retention by beach carbonate debris have also been carried out. The data suggest that sedimentary arsenic occurs largely bound to particles of the calcareous red alga Corallina panizzoi, whereby live specimens contained much smaller amounts of this metalloid than was the case for nonliving material (2.4 and 20.3 mg kg−1,respectively). Experimental tests confirmed the ability of C. panizzoi detritus to retain arsenic at pH intervals and ionic strength characteristic of seawater. There are two potential sources of that metalloid for calcareous debris in sediments: brown macroalgae, which were found to contain high levels of As (up to 66.3 mg kg−1), and ferruginized sandstones (up to 23.0 mg kg−1). We argue that any contribution of brown algae to beach sediment enrichment by As would be minor,and consider the ferrous sandstones from coastal sedimentary rocks of the Barreiras Group as the principal largescale source of arsenic in the marine environment of Espírito Santo. The experimental data, together with field studies, corroborate the interpretation that arsenic anomalies in sediments with calcareous debris can form when weathered continental rocks even only slightly enriched in As are leached by marine waters, and the As is at least partially retained by biogenic calcareous detritus in nearshore sediments. Considering that rocks of the Barreiras Group are exposed to marine erosion far to the north of Espírito Santo, we estimate that marine sediments containing calcareous material are “anomalously” enriched in As along approximately 2,000 km of the Brazilian tropical coastline.