Holocene Coral Reef Development in the Tropical Eastern Pacific PDF Download

Author: Lauren Trent Toth
Publisher:
ISBN:
Category :
Languages : en
Pages : 804

Get Book

Book Description
In recent decades, a variety of local- to global-scale disturbances have caused the global degradation of coral-reef ecosystems. In most regions, large-scale coral mortality is a recent phenomenon. In contrast, my records from the tropical eastern Pacific (TEP) have revealed a period of limited coral-reef development, beginning about 4000 cal BP (calibrated calendar years before 1950), which lasted for the next 2500 years. This protracted hiatus in reef growth provides the opportunity to test explicit predictions about the causes of reef collapse, the controls on reef resilience, and the likely trajectories of reef development in the future. Reefs of the TEP are subject to a variety of modern environmental stressors, the most important of which are seasonal upwelling and interannual impacts of the El Niño-Southern Oscillation (ENSO); however, the relative roles of these stressors in shaping past reef development are unknown. Although it is generally accepted that reef growth in the TEP is slow compared with other regions around the world, this assumption has never been tested, and the paleoecological record from reefs in this region is limited. Using data from cores of reef frameworks collected throughout Pacific Panamá, this study evaluated trends in reef development from ~7000 cal BP to the present to assess the impacts of environmental variability on reef growth during the Holocene. To further characterize the observed hiatus in reef growth and investigate the impact of seasonal upwelling on past reef development, I compared trends in vertical reef accretion among three sites situated along a contemporary gradient of seasonal upwellling: Contadora Island, which received the most intense seasonal upwelling; Iguana Island, where upwelling was moderate; and Canales de Tierra Island, where there was no upwelling. By reconstructing the age of the local radiocarbon reservoir for each of these sites, I confirmed that this general upwelling regime existed from at least ~7000 cal BP to the present. I also used geochemical analyses of fossil coral skeletons and reef sediments from Contadora to reconstruct the regional climatic and oceanographic conditions over the last 7000 years and evaluated their impacts on coral condition. Coral reefs at all three sites stopped growing from ~4100-1600 cal BP, coincident with the shutdown of reef development at several other locations elsewhere in the Pacific. There was a dramatic and significant reduction in reef accretion during the depositional hiatus. Contrary to earlier surmises that reef growth was slow in the TEP, however, millennial rates of reef accretion in Pacific Panama ́before and after the hiatus were similar to those documented in the Caribbean region. There was no significant difference in reef accretion among sites, which suggests that although upwelling has an impact on the short-term growth of corals, upwelling does not impact long-term reef accretion. Upwelling did, however, have a significant impact on the duration of reef collapse. The hiatus in reef accretion began earlier, ended later and, therefore, lasted significantly longer at Contadora, where reefs are subject to the strongest seasonal upwelling. Although upwelling was most likely not the ultimate cause of the hiatus, persistent upwelling may reduce the capacity of a reef to recover from major disturbances. Correlative evidence from regional paleoclimate proxies suggested that the ultimate cause of the hiatus was most likely enhanced climatic oscillations, particularly variability associated with ENSO. In the TEP, El Niño and La Niña events result in dramatic changes in sea temperatures, nutrient inputs, turbidity, and sea level, all of which can be problematic for reef development. ENSO activity increased 4500-4000 cal BP, coincident with the onset of the hiatus. Additionally, ENSO variability was likely enhanced from 4000-2000 cal BP due to it becoming in-phase, or "coupled", with the Intertropical Convergence Zone (ITCZ). Overall, hiatus was a time of stronger and more frequent El Niño and La Niña events and this period of enhanced climatic variability likely overwhelmed the capacity of reefs to recover. Reef growth resumed when there was a change in the mode of ENSO, with more El Niño events, but fewer La Niña events after ~2000 cal BP. Climatic reconstructions from this study suggested that oceanic productivity at Contadora was higher in the mid-Holocene, 6000-4000 cal BP, likely as a result of increased upwelling, compared with the period 4000-2000 cal BP. These results supports the conclusion of other researchers that the early to middle Holocene, may have been more "La Niña-like", compared with the late Holocene, when mean climate had shifted to a more "El Niño-like" state. Furthermore, my environmental reconstructions before and after the hiatus in reef accretion reveal that climatic and oceanographic variability in the TEP were greater over centennial scales than previously surmised. From 5000-4500 cal BP, conditions were warmer and drier, and there was less upwelling, than at other times in the Holocene. Oceanic productivity and climatic variability both increased in the 100-200 yr preceding the shutdown of reef growth. These changes are consistent with the hypothesis of escalating ENSO variability overall, and in particular an increase in La Niña activity. I used coral [lower case delta]13C, a measure of productivity and coral health, and B/Ca, a measure of carbonate saturation state within the coral, to track changes in coral condition through time. Dramatic declines in both coral productivity and coral saturation state occurred just before the hiatus, indicating that sublethal declines in coral health could serve as early warning signs of ecosystem collapse. Although coral productivity was high just after the hiatus, it gradually declined over the period ~1500 cal BP to present. One explanation for the deteriorating health of corals during this interval was that the increase in the number of El Niño events through the late Holocene resulted in an increasing frequency and severity of coral bleaching events. The parallel between the reduction in coral productivity before the collapse of Panamanian reefs ~4100 cal BP and the slow decline of coral productivity through the late Holocene may indicate that reefs in the TEP are headed toward another significant interruption in reef development in the future. There is no clear consensus on how anthropogenic climate change will affect ENSO in the future; however, many of the same conditions that occurred during the hiatus are also expected under reasonable scenarios of future climate change. If the history of reef development in Pacific Panamá is any indication of what may happen in the future, climate change could indeed be pushing some Pacific reefs towards another major collapse. On the other hand, these reefs were able to recover after a 2500-yr hiatus in reef development. Thus, if the current trajectories of climate change can be reversed, there may yet be hope for the reefs of the future.