Burial Rate Determines Holocene Rhodolith Development on the Brazilian Shelf

Abstract

The structure and composition of rhodoliths in two regions of the Brazilian shelf, Abrolhos Continental Shelf (ACS) and South Esp´ırito Santo State (SES) were examined and compared. Rhodoliths were sampled at depth ranges of 10-20 m and 50-60 m in SES, and 20-30 m and 50-75 m in ACS. Rhodoliths in SES are algal boundstones, built mainly of melobesioid corallines, with subordinate bryozoans and encrusting foraminifers. They show high porosity and the sediment infill of borings and voids contains a relatively high amount of siliciclastics (up to 8%). Rhodoliths from ACS are formed by a structureless carbonate mass covered by a thin veneer of encrusting coralline algae. The massive interior was produced by multiphase boring and infilling of an original boundstone. The infillings consist of a micritic matrix with bioclasts and low amounts of siliciclastic grains. Coralline assemblages are reduced to fragments of thin crusts. Rhodoliths from shallow depths are small (, 8 cm) whereas rhodoliths from the deeper zone have a wide size range (1 to 17 cm). The innermost parts of deeper rhodoliths in ACS yield radiocarbon ages of ~ 7,000 years BP (75 m) and ~ 2,000 years BP (65 m). Rhodoliths from the deep zone in SES are younger (less than 700 years BP). Siliciclastic sediment influx reaching rhodolith beds promotes burial of rhodoliths, determining the small size of shallow SES rhodoliths and the relatively young ages of the deeper ones. Scarce siliciclastic influx at the ACS rhodolith beds favors long residence times of rhodoliths on the seafloor, resulting in a thorough destruction of the original coralline/invertebrate boundstone by bioerosion.Despite the increasing knowledge about Brazilian rhodoliths in the last decade, little is known about their inner arrangement and composition (Leal et al. 2012; Tamega et al. 2014; Vale et al. 2018). The taxonomic composition, morphology, inner structure, and interactions of coralline algae with other encrusting groups (such as bryozoans, corals, and serpulids) are relevant for understanding the factors that control long-term development of rhodoliths (Adey and MacIntyre 1973; Aguirre et al. 2012) and to reconstruct the paleoenvironments in which they grew (Bosence 1983a;

Structure of Rhodolith Beds and Surrounding Habitats at the Doce River Shelf (Brazil)

“…However, we found terrigenous sediments trapped on rhodoliths and carbonate concretions in this area. Following Brasileiro et al [17] and Vieira et al [32] and the results obtained herein, we can affirm that the Doce River sedimentation affected these habitats, however, we did not determine yet that these sediments originated from mining wastes. In contrast, at another MPA, Santa Cruz National Wildlife Refuge, rhodoliths were absent in shallower waters, probably due to a high sedimentation regime from the local river Piraquê-Açu and low water motion.…”

“…In our study, the rhodoliths from 50 m depth were smaller (~4 cm) than those sampled at the south of ESCS (ca. 8 cm; [13]) or at the Abrolhos Bank (mean 6 cm; [14]) also due to sedimentation as observed by Brasileiro et al [17] by analyzing their inner structure. Ultimately, the mean coverage and small size of rhodoliths at the study area may also result in a lower carbonate production than the Abrolhos Bank (estimated in 1.0 ± 0.7 kg m −2 yr −1 , [14]).…”

“…The world´s largest RBs were reported in the Brazilian Continental Shelf, where they represent a major marine habitat over extensive and continuous areas from 2 • N to 23 • S, encompassing the mid-to outer-shelf (30-120 m depth) and surrounding oceanic islands [6]. Several aspects about these beds have been investigated such as, for example, their distribution, mapping, associated organisms, coralline species composition, and internal structure [6,[13][14][15][16][17][18]. RBs and carbonate reefs off the Amazon River mouth have recently drawn international attention because of their high associated biodiversity coupled with their proximity to allotments of Oil and Gas block exploration fields in the Brazilian Equatorial Margin [18,19].…”

Middle Eocene Rhodoliths from Tropical and Mid-Latitude Regions

“…The largest rhodolith beds in tropical latitudes occur nowadays on the eastern Brazilian shelf [84,85], in areas with relatively reduced development of coral reefs [86]. Here, rhodolith beds extend from shallow subtidal settings to the shelf margin [84,85,87,88] and thrive under mesotrophic conditions, with mean seawater temperatures higher than 20 • C on the sea floor [88][89][90], and low terrigenous sedimentation, which is generally limited to near-shore areas [91]. Similarly, extensive rhodolith beds are found in the Amazon River mouth in the northwestern Brazilian platform, associated to the so-called Great Amazon Reef System [92].…”

“…temperatures higher than 20 °C on the sea floor [88][89][90], and low terrigenous sedimentation, which is generally limited to near-shore areas [91]. Similarly, extensive rhodolith beds are found in the Amazon River mouth in the northwestern Brazilian platform, associated to the so-called Great Amazon Reef System [92].…”