Holocene Reef Development Along the Northeastern St. Croix Shelf, Buck Island, U.S. Virgin Islands

Abstract Eight cores were recovered from Buck Island Underwater National Monument (U.S. Virgin Islands). Facies were defined based on recovered coral species, fabrics observed in core slabs and thin sections, and detailed notes on drilling character. Thirty-six radiometric dates constrained the timing of reef accretion. Together, these data provide a detailed history of reef development under varying regimes of sea-level rise and physical oceanography. Holocene reefs around Buck Island initiated atop a broad antecedent bench at 13–16 meters below present sea level. Shelf flooding near Buck Island occurred as early as 9,500 years ago (CalBP), but preserved reefs lagged by as much as 1,800 years. Earliest reef development was dominated by branching Acropora palmata near the shelf edge and massive corals closer to Buck Island. By 7,200 CalBP, A. palmata apparently declined near the platform margin and was absent until ca. 5,200 CalBP throughout the study area. Over time, the reefs closer to Buck Island built upward (ca. 16 m) and seaward (ca. 50 m), as the rate of sea-level rise slowed and carbonate production increasingly exceeded the accommodation space that was being created. Reef topography and zonation became progressively more distinct, with A. palmata dominating the shallow reef crest. Branching coral apparently disappeared again between ca. 3,030 and 2,005 CalBP for reasons that are not clear. This and the previous decline of A. palmata mimic patterns seen around St. Croix and throughout the Caribbean. By 1,000 CalBP, the reefs close to Buck Island had largely assumed their present character and continued to track slowly rising sea level until the present. Around 1,200 CalBP, vertical accretion along Buck Island Bar apparently ceased. Paradoxically, the surface of this outer reef has historically been dominated by large stands of A. palmata since the area was first described, but rapid coral growth has not resulted in preservation of this species over the last millennium. Modern community structure mimics facies patterns seen in cores. Over the past 7,700 years, the southern reef crest appears to have remained slightly shallower than its northern counterpart, a condition that persists today. Observations after Hurricane Hugo in 1989 suggest that this difference in elevation is related to the piling up of debris on the broader, southern reef crest by high waves from storms passing south of St. Croix. Also, facies along the southern reef are more variable in species composition than their northern counterparts, a condition that is exhibited by the modern reef community. Coral abundance and diversity in the cores (total coral = 20–30%; dominated by A. palmata) are comparable to the community structure present in the late 1970s (Bythell et al. 1993; Hubbard et al. 1993). In contrast, fossil-coral abundance and diversity are consistently higher than what was measured in the 1980s and early 1990s (total coral = 7–14%; A. palmata ≤ 2%), after the onset of White Band Disease, a putative pathogen, which has recently decimated branching acroporids throughout the region. The dominance of branching A. palmata in the cores would seemingly reflect an absence of disease or other factors that would discourage its continued abundance. In apparent contrast, two lengthy gaps in the A. palmata record reflect previous disappearances that roughly correspond to similar lapses elsewhere in the Caribbean. Thus, the spatial persistence of a species in the fossil record cannot necessarily be equated with its temporal continuity. Comparisons between changes in modern reefs on a time scale of decades and their fossil forebears must be made with great care. Understanding the role of short-term changes and how they are reflected in the preserved record is thus critical to relating the late Holocene A. palmata gaps to the recent decline of the species. This has important implications for our understanding of how preserved community structure relates to what actually existed in the past, and could limit our ability to use the recent geologic record as a proxy to short-term, future changes in coral reefs.