Foraminiferal communities of a mid-Holocene reef: Isla Colón, Caribbean Panama

Highlights

• Foraminiferal distribution suggests a patch reef facies rather than a framework.

• Foraminiferal taxa from seagrass and proximal reef facies are most similar.

• A. parkinsoniana differentiates the muddier habitats from reef or seagrass facies.

Abstract

The distribution of mid-Holocene, tropical benthic foraminiferal assemblages, including species' proportions, diversity, dominance and wall type, were analyzed for their correspondence to marine habitats and invertebrate facies. Benthic foraminifera are useful for paleoenvironmental reconstructions because the modern ecology of many species found as Quaternary fossils is known. Samples were collected from trenches in ~30,000 m² of an excavated Acropora cervicornis-dominated, mid-Holocene reef with an age of ~6 kyr on Isla Colón (Colon island), bordering Almirante Bay in Bocas del Toro, Caribbean Panama. Bulk sediment samples were collected from a maximum depth of ~7 m below modern mean sea level and classified in the field into five invertebrate biofacies: 1) A. cervicornis-dominated reef, 2) molluscan mud, 3) Porites-Agaricia reef, 4) mixed coral, and 5) Lobatus-dominated seagrass. Sediment carbon and grain size analyses, a cluster analysis performed on the relative abundance of species per sample, and the Fisher's alpha diversity index were used to compare sample similarity and environmental variables to determine habitat relationships. Most samples contained high total inorganic carbon and poorly sorted, medium-coarse sediments. Principle component analysis of sediment grain size and carbon values did not show a clear correspondence among samples, habitat type or location of trenches. Foraminiferal assemblages in A. cervicornis and other reefal samples (categories 1, 3 and 4, above) had the greatest diversity and did not distinguish between the three reefal types, suggesting similar, normal marine conditions and/or mixing of coral fragments. Molluscan mud samples with high total organic carbon content were least diverse with dominant Ammonia and Elphidium taxa, though foraminifera in other molluscan mud samples showed a transition to proximal reef corals. Seagrass samples were differentiated from molluscan mud samples and had similar diversities and species assemblages to the Porites-Agaricia samples, and taxa known to be temporary grazers and living temporarily attached to seagrass blades are present in relatively greater amounts in these samples.

Based on the distribution of foraminiferal species across this mid-Holocene coral reef, we conclude that it was a patch reef that included high-organic and seagrass facies similar to those of modern Almirante Bay. Results from this study can be compared to modern foraminiferal studies to investigate whether the modern habitats of BDT are significantly different from the pristine reefs of the mid-Holocene.

Introduction

Roughly 80% of all Caribbean coral cover has declined since the 1970s (Gardner et al., 2003; Jackson et al., 2014) and although human activity is the suspected main cause for these recent changes, extinctions and turnovers of reef species have occurred for millions of years. For example, Plio-Pleistocene glacial cycles influenced changes in ocean circulation patterns and marine conditions in the Pacific and Caribbean, resulting in a turnover of coral species to those better adapted to survive climatic changes (Budd and Johnson, 2010). Most modern declines in reef health are attributed to disease, loss of herbivores through overfishing that has resulted in macroalgal overgrowth of corals, and high sedimentation, resuspension and eutrophication (Lessios, 1988; Mumby et al., 2007; Mumby and Steneck, 2008). In Bocas del Toro, Caribbean Panama, increased runoff and changes in coral cover from domination of Porites to Agaricia suggested that water quality in the region has been degraded, prompting additional monitoring of conditions such as water clarity, eutrophication, temperature stress and the effects of pollutants on reefs and regional marine biodiversity (Collin et al., 2009; Berry et al., 2013; Seemann et al., 2014). However, the pre-altered, Quaternary state of coral reef ecosystems in this region has been unknown until recent studies that have shown they are distinctly different from the modern reefs (Cramer et al., 2020; O'Dea et al., 2020). This study of benthic foraminiferal assemblages from “pristine,” mid-Holocene coral reef facies in Bocas del Toro provides insight into the diversity and distribution of these assemblages prior to human disturbance.

Benthic foraminifera, marine protists that produce either a calcium carbonate or agglutinated test of cemented sand, organic matter or other marine particles, are sensitive to temperature, salinity and water depth, making species useful as environmental indicators. The proportion of hyaline, porcellaneous and agglutinated wall types within foraminiferal assemblages has been used as a proxy for past and modern habitat salinity. In warm-water, carbonate-dominated environments, the distribution of porcellaneous, hyaline and agglutinated species tends to vary according to salinity and distance from shore; brackish-to-normal marine lagoons are dominated by hyaline species, while normal marine lagoons have a near-even mix of hyaline and porcellaneous species, with porcellaneous species dominating in hypersaline lagoons (Murray, 2006). For these reasons, foraminifera have been used to reconstruct past environments to identify changes in local and global marine conditions, such as the effects of habitat loss and declining water quality (Debenay and Fernandez, 2009). Many studies have shown that some species have an association with sediment grain size (Bock, 1971; Boltovskoy and Wright, 1976; Havach and Collins, 1997), which may depend on the amount of organic matter (Poag, 1981; Brasier, 1975a), water energy and ocean circulation. For example, Brasier (1975a) reported that studies from the Caribbean (Cebulski, 1969; Wright, 1964; Seiglie, 1971) suggested that stable backreef and bay faunas yield thinly walled miliolids and soritids in association with clear, calm water, and abundant carbonate fines can be found in greater abundance in seagrass habitats. However, in the Bocas del Toro region across 3.7- to 240-m water depths, the correlation between grain size and habitat tends to be weak (Havach and Collins, 1997).

In this study we investigate the distribution and diversity of benthic foraminiferal assemblages from a ~ 6000-year-old coral reef with associated seagrass and mollusk habitats that grew under a mixed carbonate-siliciclastic regime near the island of Isla Colón, Bocas del Toro, Panama. This study addresses the following questions: 1) How does the benthic foraminiferal distribution and diversity compare across mid-Holocene, tropical, reefal habitats?; 2) How do the sampled habitats compare to sediment grain size and carbon measurements?; and 3) How do the foraminiferal associations compare to field assessments of invertebrate-associated habitats?

Pleistocene uplift of the Bocas del Toro (BDT) archipelago (Collins et al., 1995) and rising seas over the last ~8 kyr (Lambeck et al., 2014) caused formation of its many islands (Fig. 1). The Bocas del Toro Province (8° 55′- 9° 30′ N, 82° 15′- 81° 20′ W) is sedimentologically unusual in the Caribbean because of its mixed carbonate-siliciclastic regime that results from erosion of siliciclastics from the nearby Central Cordillera, within eyesight of the BDT coastline, combined with modern coral rubble and carbonate shoals in the surrounding coastal areas. Isla Colón, the largest island within the archipelago, and the location of this mid-Holocene reef study, is bounded by the open Caribbean Sea to the northeast and Almirante Bay to the southwest.

The mid-Holocene reef was formed before human settlement along the coast, so has only been altered by natural changes. Though the human occupation of Panama dates back to the terminal Pleistocene (11,100–10,000 cal yr B.P.), radiometric dates from two archeological sites (Fig. 1) show occupation in Sitio Drago, northwest corner of Isla Colón, by 690–1410 CE, and Cerro Brujo, on the Aguacate Peninsula, around 880–1250 CE (Linares, 1977; Linares and Ranere, 1980). Thus, the preserved reef provides an ideal, “pristine” setting for reconstructing and differentiating past habitats using environmentally responsive benthic foraminiferal assemblages.