Michael R. Toomey

Profiles of ocean island coral reefs controlled by sea-level history and carbonate accumulation rates

Modern and preserved coral reefs on islands exhibit a broad range of forms, from actively accreting fringing and barrier reefs to terraces preserved by drowning or subaerial exposure. Darwin’s canonical model of reef development proposes an evolutionary sequence of reef forms as a volcanic island ages and subsides, from fringing reef to lagoon-bounding barrier reef to atoll. Compiled data from modern systems show, however, that many islands do not follow this sequence, implying that reefs are shaped by more than island subsidence alone. We show that the diversity of modern reef morphology arises from the combined effects of island subsidence, coral growth, and glacial sea-level cycles. A model for the evolution of a reef elevation profile over the past 400 k.y. reveals that different combinations of reef accretion rate and island vertical motion produce a variety of forms that matches the observed distribution of modern reefs. This match occurs only if the model is driven by Pleistocene sea-level oscillations—few modern environments have the right combination of conditions to produce the Darwinian atoll progression.

The intertropical convergence zone modulates intense hurricane strikes on the western North Atlantic margin

Most Atlantic hurricanes form in the Main Development Region between 9°N to 20°N along the northern edge of the Intertropical Convergence Zone (ITCZ). Previous research has suggested that meridional shifts in the ITCZ position on geologic timescales can modulate hurricane activity, but continuous and long-term storm records are needed from multiple sites to assess this hypothesis. Here we present a 3000 year record of intense hurricane strikes in the northern Bahamas (Abaco Island) based on overwash deposits in a coastal sinkhole, which indicates that the ITCZ has likely helped modulate intense hurricane strikes on the western North Atlantic margin on millennial to centennial-scales. The new reconstruction closely matches a previous reconstruction from Puerto Rico, and documents a period of elevated intense hurricane activity on the western North Atlantic margin from 2500 to 1000 years ago when paleo precipitation proxies suggest that the ITCZ occupied a more northern position. Considering that anthropogenic warming is predicted to be focused in the northern hemisphere in the coming century, these results provide a prehistoric analog that an attendant northern ITCZ shift in the future may again return the western North Atlantic margin to an active hurricane interval.

Increased hurricane frequency near Florida during Younger Dryas Atlantic Meridional Overturning Circulation slowdown

15 The risk posed by intensification of North Atlantic hurricane activity remains 16 controversial, in part due to a lack of available storm proxy records that extend beyond 17 the relatively stable climates of the late Holocene. Here we present a record of storm18 triggered turbidite deposition offshore the Dry Tortugas, south Florida, USA, that spans 19 abrupt transitions in North Atlantic sea-surface temperature and Atlantic Meridional 20 Overturning Circulation (AMOC) during the Younger Dryas (12.9–11.7 k.y. B.P.). 21 Despite potentially hostile conditions for cyclogenesis in the tropical North Atlantic at 22