COLORED SHELL HISTORIES: TRACKING TAPHONOMIC PATHWAYS OF BRACHIOPOD SHELLS IN SHALLOW-WATER SETTINGS OF CARAGUATATUBA BAY, SÃO PAULO BIGHT, SOUTHEASTERN BRAZIL

Recent studies on bivalve mollusks have demonstrated that postmortem shell discoloration can act as a “taphonomic clock,” providing insights into the minimum age of shells and the environmental conditions (i.e., taphonomic pathways) they experienced. This approach is gaining traction in both Paleontology and actualistic Taphonomy. Here, we investigate color alteration in brachiopod shells from Caraguatatuba Bay, located in the São Paulo Bight on the southeastern Brazilian continental shelf. Samples were collected in September 2001 using a dredge (30 × 40 cm, 5 mm mesh) along 2 km transects at varying depths. Additional material was obtained with a Van Veen grab sampler. A dense brachiopod accumulation was identified at a depth of 20 meters (45º12′18″ W, 23º40′1″ S) in muddy siliciclastic sediments. The assemblage is dominated by thick-walled, punctate, low-organic, calcitic shells of Bouchardia rosea (Mawe, 1823), a free-lying, epifaunal suspension feeder known to form extensive bioclastic deposits on the Brazilian shelf. Of the 337 shells analyzed (284 from dredging, 53 from grab sampling), 83.1% were gray, 16.0% brown, and only 0.89% white (bleached). Gray shells show advanced dissolution and surface etching, while brown shells appear largely intact. SEM/EDS and electron microprobe analyses revealed iron infilling in the punctae of both gray and brown shells, along with traces of silica, titanium, and manganese (the latter detected only via microprobe). A pyrite crystal was observed on the exterior of one brown shell. These results suggest that gray shells underwent more intense or prolonged alteration under reducing conditions. In contrast, the preservation of brown shells implies exposure to oxidizing settings without prior deep dissolution. Therefore, gray and brown shells likely represent distinct taphonomic pathways rather than sequential stages of alteration. The dominance of gray shells supports long-term residence in reducing environments (e.g., sulfate-reduction zones), consistent with iron sulfide formation and organic enrichment, conditions known to promote millennial-scale postmortem shell modification in marine assemblages. Despite color differences, all shells shared similar mineralogical and elemental compositions, indicating that discoloration stems from external geochemical processes rather than intrinsic shell properties.