Kristin E. Riker-Coleman

Drowning of the -150 m reef off Hawaii: A Casualty of global meltwater pulse 1A?

We present evidence that the drowning of the 2150 m coral reef around Hawaii was caused by rapid sea-level rise associated with meltwater pulse 1A (MWP-1A) during the last deglaciation. New U/Th and 14 C accelerator mass spectrometry dates, combined with reinterpretation of existing radiometric dates, constrain the age of the coral reef to 15.2-14.7 ka (U/Th age), indicating that reef growth persisted for 4.3 k.y. following the end of the Last Glacial Maximum at 19 ka. The drowning age of the reef is roughly synchronous with the onset of MWP-1A between 14.7 and 14.2 ka. Dates from coralline algal material range from 14 to 10 cal ka (calibrated radiocarbon age), 1-4 k.y. younger than the coral ages. A paleoenvironmental reconstruction incorporating all available radiometric dates, high-resolution bathymetry, dive observations, and coralgal paleobathymetry data indicates a dramatic rise in sea level around Hawaii ca. 14.7 ka. Paleowater depths over the reef crest increased rapidly above a critical depth (30-40 m), drowning the shallow reef-building Porites corals and causing a shift to deep- water coralline algal growth, preserved as a crust on the drowned reef crest.

Coralgal composition of drowned carbonate platforms in the Huon Gulf, Papua New Guinea; Implications for lowstand reef development and drowning

Abstract Collision between the South Bismarck plate and the northern edge of the Australian plate has produced an actively subsiding foreland basin in the western Huon Gulf. A series of drowned carbonate platforms and pinnacles are preserved on this margin due to a combination of this rapid subsidence and eustatic sea-level changes over the last 450 ka. We analyzed sedimentary and coralgal data from the platforms to better understand lowstand reef development and drowning in the Huon Gulf. The recovered limestones are divided into five main sedimentary facies: coral reef, coralline–foraminiferal nodule, coralline–foraminiferal crust, Halimeda , and planktonic foraminiferal limestones. Based on a comparison with modern analogues in the Indo-Pacific and elsewhere, we identified coral reef, deep fore-reef slope, deeper fore-reef slope, and pelagic/hemipelagic paleoenvironmental settings. An analysis of facies relationships and their paleoenvironmental meanings revealed lowstand corals reefs preserved at the top of the platforms that grew within ∼10 m of sea level. Two different coral assemblages were identified within this facies: (1) a shallow, high energy reef community characteristic of windward margins and limited to the deep platforms (1947, 2121, 2393 m), and (2) another shallow community but indicative of more moderate lower energy reef conditions and limited to the middle (1113, 1217, 1612 m) and shallow platforms (823 m). The change from high to lower energy reef growth conditions suggests that oceanographic/climatic conditions in the Huon Gulf have changed substantially through time, primarily through the closure of the Gulf as a result of tectonic rotation and uplift of the Huon Peninsula over the last 450 ka. Despite major environmental perturbations (i.e. relative sea-level and temperature changes) the platforms and the shallow water coral reefs exposed at the top have been able to re-establish themselves time and time again over the last 450 ka. We also identified two different incipient drowning scenarios influenced by the rate of relative sea-level rise. More rapid drowning in the middle and deep platforms produced a thin veneer of coralline–foraminiferal nodule and Halimeda limestones over the shallow coral reef material while the slower drowning experienced by the shallowest platforms allowed thick coralline–foraminiferal crust limestones to develop. We recognize three main stages of platform development: (1) initiation and growth characterized by shallow coral reef growth as the platforms grew close to sea level during the lowstands, (2) incipient drowning marked by a shift to coralline–foraminiferal nodule, crust and Halimeda limestones as the platforms began to drown during rapid eustatic sea-level rise and continued subsidence, and (3) the complete drowning of the platforms characterized by platform ‘turn off’, increased bioerosion, Fe–Mn precipitation and pelagic/hemipelagic sedimentation as the platform surfaces finally drop below the photic zone.

Earth's climate: Past and future

Given the prominence of global climate change issues in the United States and abroad, it is surprising that there has not been an introductory-level textbook that adequately addresses the Earths climate system in a systematic, easy to read, multidisciplinary manner. Earths Climate: Past and Future by W. F. Ruddiman is exactly that: an excellent introductory textbook covering the basic science and the paradigms that govern our current understanding of Earths climate system. In his opening remarks, Ruddiman states his goal that the book make climate science interesting and intelligible to all students, especially to non-science majors taking an upper-division science course. Ruddiman successfully tackles this goal.