ST. CROIX REEF HISTORY REVISITED: INIMICAL WATERS TURNED FRIENDLY

Seventeen cores across the St. Croix shelf record a history of Holocene ref development that started near the eastern platform margin 10,600 years ago and was dominated by branching Acropora palmata. Reefs generally started on an antecedent topographic rim around Lang Bank to the east at a depth of 20-24 m below present sea level. Despite water depths greater than 10 m, branching-coral reefs continued uninterrupted until roughly 6,400 ybp and were gradually closing the gap on rapidly rising sea level when they abruptly shut down. The 4,200 year record of continuous reef accretion on Lang Bank is contrary to previous presumptions that reefs all around the Bank (and perhaps Caribbean-wide) in 10-30 meters were killed off 10,000 years ago by turbid water as rising sea level broke over the shelf edge and mobilized soil horizons created during the last low-stand. While “inimical bottom waters” remain as a viable mechanism for discouraging reef accretion, the mechanism appears to ironically not have occurred at the site where this concept was first described.

After 6,400 ybp, the main sites of active reef development were located closer to St. Croix and Buck Island, where the primary reef builders were massive corals, principally Montastrea spp. Previous studies have shown that reefs on the north coast of St. Croix started from a shifting sandy bottom around 7,000 ybp and built vertically until they caught up with rising sea level. To the north, massive-coral reefs around Buck Island started between 7,500 and 7,000 years ago on antecedent platform 12-15 m below present sea level. The reefs continued to build both upward and seaward, gradually shifting back to A. palmata as they caught up with sea level.

This example of backstepping is analogous to Devonian sequences in Canada and Australia. Also, both the abandonment of shelf-edge reefs and the rapid decline of A. palmata are similar in character and timing to the reef history along the southwest coast of Puerto Rico and Florida. This scenario argues against increasingly popular assumptions that large-scale community shifts seen in recent monitoring records are unprecedented in the Holocene and Pleistocene record. The Holocene reefs of St. Croix, therefore, have valuable applications both backward and forward through geologic time.