TIMING AND DURATION OF THE LAST INTERGLACIAL PERIOD FROM U-SERIES AGES OF UNALTERED REEF CORALS, NORTHERN AND WESTERN PUERTO RICO

Although there have been many studies of last-interglacial (LIG) shorelines (emergent reefs and terraces), there are still uncertainties about the timing and duration of LIG sea level elevations near or above the present sea level. The island of Puerto Rico has probably experienced slow, long-term uplift in the late Quaternary (on the order of 0.03-0.05 m/ka) and therefore sea level records need to be interpreted with caution. Nevertheless, LIG reef tract deposits are common along much of the northern and western coasts of Puerto Rico and contain well preserved corals (95-100% aragonite). We analyzed corals from localities near Palmas Altas, El Vigia, Punta Agujereada and Punta Higüero for U-series dating. Corals in growth position are common in most reef tracts and are found at elevations between about 0.5 and 1.5 m above high tide level. Of 19 LIG corals analyzed, all have U contents similar to their modern counterparts and low common Th contents. Fifteen of the corals have back-calculated initial 234-U/238-U activity ratios of 1.140 to 1.154, overlapping the range of values found in modern seawater. These 15 samples indicate probable closed-system histories with respect to U-series nuclides and yield ages with a high degree of confidence. Ages range from 127.7 ± 0.7 ka to 114 ± 2 ka (two-sigma errors); one sample gave an age of 105.0 ± 0.5 ka. In the Holocene, sea level rose to present levels several thousand years after the high-latitude, Northern Hemisphere summer insolation peak at ~11 ka. The Puerto Rico coral data presented here indicate that sea level was near or above present at the time of the previous summer insolation peak at ~127 ka. Further, the Puerto Rico data indicate that the LIG high sea level probably lasted a minimum of 12 ka. This contrasts with the deep-sea oxygen isotope record, wherein the LIG high sea stand is thought to have had a duration of only about 7 ka at most.