CORAL HIGH-RESOLUTION METAL/CA RECORDS FROM TODOS OS SANTOS BAY: A MULTI-PROXY TOOL FOR ENVIRONMENTAL RECONSTRUCTION

The Todos os Santos Bay (BTS) is one of Brazil's largest bays and plays a significant role in the regional development of the state of Bahia. Historically, the BTS has undergone numerous transformations, beginning with the Portuguese colonization and the establishment of the region's first urban centers. Industrialization intensified in the mid-20th century with the development of major industrial hubs such as the Aratu Center, the Camaçari Petrochemical Complex, and the Subaé Center. The intensive development of industry, tourism, fishery, and unbanization in the BTS region has contributed to increased loads of potentially toxic metals in the marine environment, posing threats to ecosystem functioning and representing ecotoxicological and public health risks. To monitor and understand the evolution of these environmental impacts, several approaches have been employed, including the analysis of sediment cores and the monitoring of the physicochemical characteristics of seawater. However, such methods generally lack high temporal resolution, particularly on monthly or biweekly scales. As an alternative to the scarcity of historical data, natural archives such as corals offer ideal characteristics for long-term environmental assessment. This study aims to reconstruct the temporal variability of metal/Ca ratios along a coral core from the species Siderastrea stellata (24-BTS-SS1), collected from the BTS in August of 2024. The core 24-BTS-SS1 was cleaned with deionized water and subsequently, a 5 mm-thick coral slab was cut and radiographed for density banding analysis. The chronology of the core was established by counting high- and low-density bands observed in the radiographs, using the CoralXDS software. High resolution carbonate sampling was performed with a spatial resolution of 0.5 mm at the surface and 1.0 mm in depth using a precision micromill. Metal/Ca ratios were measured using inductively coupled plasma mass spectrometry (ICP-MS), targeting the isotopes 55Mn, 59Co, 60Ni and 208Pb normalized to 43Ca. Analytical control was assured through repeated measurements of three certified coral reference materials: Ishi-B, JCp-1 and NIST Coral. Analytical uncertainty (in % RSD) was better than 8 % for all metal/Ca ratios. Annual density band analysis of coral 24-BTS-SS1 revealed a 95-year record, spanning from 1929 to 2024. The average annual growth rate of the colony was 3.3 mm.year-1. Preliminary analyses of metal/Ca ratios for the last 20 years (2000–2024) showed the following ranges (in µmol/mol) and mean (± standard deviation): Ni/Ca (0.57–2.34; mean 0.75 ± 0.26), Co/Ca (0.68–0.85; mean 0.74 ± 0.03), , Pb/Ca (0.09–11.56; mean 0.67 ± 1.10), and Mn/Ca (1.08–18.89; mean 2.11 ± 1.61). The variability and anomalies for the different metal/Ca do not exhibit similar pattern over the 20-year record, suggesting multiple pollution sources for the different elements. Pb/Ca, for instance, displayed a sharp peak in 2005, with smaller anomalies in 2007, 2008, and 2018. Co/Ca, in turn, showed an increasing trend from 2015 onward, consistent with patterns observed in sedimentary records from the bay. The analysis of metal/Ca content from 1929 to 2000 in coral 24-BTS-SS1 will facilitate a more comprehensive evaluation of the historical patterns of pollution in BTS. We also draw attention to the potential of the species S. stellata as a natural archive for generating high-resolution records of pollutants on the Brazilian coastline.