Geology topics

Richard B. Aronson

Publications and source records attributed to Richard B. Aronson.

15 recordsLinked to original sources

USGS2023

The potential for coral reef restoration to mitigate coastal flooding as sea levels rise

The ability of reefs to protect coastlines from storm-driven flooding hinges on their capacity to keep pace with sea-level rise. Here, we show how and whether coral restoration could achieve the often-cited goal of reversing the impacts of coral-reef degradation to preserve this essential function. We combined coral-growth measurements and carbonate-budget assessments of reef-accretion potential at Buck Island Reef, U.S. Virgin Islands, with hydrodynamic modeling to quantify future coastal flooding under various coral-restoration, sea-level rise, and storm scenarios. Our results provide guidance on how restoration of Acropora palmata , if successful, could mitigate the most extreme impacts of coastal flooding by reversing projected trajectories of reef erosion and allowing reefs to keep pace with the ~0.5 m of sea-level rise expected by 2100 with moderate carbon-emissions reductions. This highlights the potential long-term benefits of pursuing coral-reef restoration alongside climate-change mitigation to support the persistence of essential coral-reef ecosystem services.

U.S. Virgin Islands, Buck Island Reef National Mon↗

USGS2023

Upwelling, climate change, and the shifting geography of coral reef development

The eastern tropical Pacific is oceanographically unfavorable for coral-reef development. Nevertheless, reefs have persisted there for the last 7000 years. Rates of vertical accretion during the Holocene have been similar in the strong-upwelling Gulf of Panamá (GoP) and the adjacent, weak-upwelling Gulf of Chiriquí (GoC); however, seasonal upwelling in the GoP exacerbated a climate-driven hiatus in reef development in the late Holocene. The situation is now reversed and seasonal upwelling in the GoP currently buffers thermal stress, creating a refuge for coral growth. We developed carbonate budget models to project the capacity of reefs in both gulfs to keep up with future sea-level rise. On average, the GoP had significantly higher net carbonate production rates than the GoC. With an estimated contemporary reef-accretion potential (RAP) of 5.5 mm year −1 , reefs in the GoP are projected to be able to keep up with sea-level rise if CO 2 emissions are reduced, but not under current emissions trajectories. With an estimated RAP of just 0.3 mm year −1 , reefs in the GoC are likely already unable to keep up with contemporary sea-level rise in Panamá (1.4 mm year −1 ). Whereas the GoP has the potential to support functional reefs in the near-term, our study indicates that their long-term persistence will depend on reduction of greenhouse gases.

Scientific Reports↗

USGS2022

Assigning causality to events in the Holocene record of coral reefs

The uncemented reef-frameworks of Pacific Panamá, which have been dominated throughout the Holocene by branching corals of the genus Pocillopora , experienced a hiatus in vertical accretion lasting c. 2300 years, beginning c. 4100 years ago. The hiatus has been attributed to an increase in variability of the El Niño–Southern Oscillation (ENSO). We tested the alternative hypothesis that the hiatus was solely the result of bioerosion, assuming an acute disturbance halted coral growth 1800 years ago (the time at which reef accretion resumed after the hiatus) and that the entire framework remained in the taphonomically active zone at that time. We calculate that it would have taken 167–511 years for bioerosion to fully remove 2300 years-worth of framework growth under those circumstances. In fact, most of the reef-framework in Panamá is stabilized in sediment that prevents the activity of bioeroders; only the upper c. 1 m of open framework – several decades-worth of growth at most – would have been vulnerable to erosion, greatly increasing the time required to bioerode 2300 years of accumulation. We conclude that the hiatus was not solely an artefact of bioerosion; rather, a long-term increase in ENSO variability suppressed coral growth and vertical reef accretion.

Journal of the Geological Society of London↗

USGS2021

Upwelling and the persistence of coral-reef frameworks in the eastern tropical Pacific

In an era of global change, the fate and form of reef habitats will depend on shifting assemblages of organisms and their responses to multiple stressors. Multiphyletic assemblages of calcifying and bioeroding species contribute to a dynamic balance between constructive and erosive processes, and reef-framework growth occurs only when calcium-carbonate deposition exceeds erosion. Each contributing species exhibits a unique combination of environmental sensitivities, trophic needs, and competitive abilities, making the net outcome of their habitat-altering behavior difficult to predict. In this study, standardized blocks of clean, massive Porites were placed at six reef sites in the eastern tropical Pacific, in the strongly and more-weakly upwelling Gulfs of Panamá (GoP) and Chiriquí (GoC), respectively. Sites were chosen to characterize the unique thermal and carbonate-chemistry conditions of each gulf. Satellite products were used to examine differences in sea-surface productivity, and surveys were conducted to quantify the abundance of important grazing taxa. After two years in situ, the Porites blocks were collected and scanned using high-resolution computed tomography to volumetrically quantify both endolithic and epilithic habitat alteration. Scan-volumes were further classified into functional groups according to morphology to quantify external bioerosion by fish and sea urchins, as well as the calcifying and bioeroding activity of crustose coralline algae, scleractinian corals, mollusks, annelids, and barnacles. The GoP, which has higher productivity, cooler temperatures, and periodically lower pH conditions, had higher rates of macroboring, but also higher rates of calcification. These unexpectedly higher rates of calcification in the GoP were a result of high recruitment of suspension-feeding taxa, particularly barnacles and vermiform fauna that have poor reef-forming potential. External bioerosion by grazers was the dominant process influencing these dead coral substrates across both gulfs, contributing to higher rates of net erosion in the GoC and underscoring the important roles that urchins and fish play in not just removing algae on reefs, but also eroding reef habitat. Ultimately these findings reveal that the trophic requirements of habitat-altering taxa are closely tied to reef-framework stability, and that environmental conditions conducive to carbonate precipitation are not necessarily those that will lead to habitat persistence.

Ecological Monographs↗

USGS2021

Climate and the latitudinal limits of subtropical reef development

Climate plays a central role in coral-reef development, especially in marginal environments. The high-latitude reefs of southeast Florida are currently non-accreting, relict systems with low coral cover. This region also did not support the extensive Late Pleistocene reef development observed in many other locations around the world; however, there is evidence of significant reef building in southeast Florida during the Holocene. Using 146 radiometric ages from reefs extending ~ 120 km along Florida’s southeast coast, we test the hypothesis that the latitudinal extent of Holocene reef development in this region was modulated by climatic variability. We demonstrate that although sea-level changes impacted rates of reef accretion and allowed reefs to backstep inshore as new habitats were flooded, sea level was not the ultimate cause of reef demise. Instead, we conclude that climate was the primary driver of the expansion and contraction of Florida’s reefs during the Holocene. Reefs grew to 26.7° N in southeast Florida during the relatively warm, stable climate at the beginning of the Holocene Thermal Maximum (HTM) ~ 10,000 years ago, but subsequent cooling and increased frequency of winter cold fronts were associated with the equatorward contraction of reef building. By ~ 7800 years ago, actively accreting reefs only extended to 26.1° N. Reefs further contracted to 25.8° N after 5800 years ago, and by 3000 years ago reef development had terminated throughout southern Florida (24.5–26.7° N). Modern warming is unlikely to simply reverse this trend, however, because the climate of the Anthropocene will be fundamentally different from the HTM. By increasing the frequency and intensity of both warm and cold extreme-weather events, contemporary climate change will instead amplify conditions inimical to reef development in marginal reef environments such as southern Florida, making them more likely to continue to deteriorate than to resume accretion in the future.

Florida↗

USGS2020

7700-year persistence of an isolated, free-living coral assemblage in the Galápagos Islands: A model for coral refugia?

In an eastern-Pacific coral assemblage at Devil’s Crown, Galápagos Islands, Ecuador, two coral species, Psammocora stellata and Cycloseris (Diaseris) distorta, form dense populations of unattached colonies on sand and rubble substrata. In the Galápagos, living C. (D.) distorta is found only at this single site, whereas populations of P. stellata are found throughout the archipelago. Six cores dating to 7700 yBP showed P. stellata to be dominant throughout the history of this isolated community, but C. (D.) distorta increased in abundance from ~2200 yBP and reached peak abundance between 1471 yBP and the present. The relative frequency of the two coral species may be linked to millennial-scale climatic variability, and this site may represent a refuge for C. (D.) distorta from unfavorable climatic fluctuations on millennial timescales. Our results demonstrate that some corals can persist in isolated populations for millennia.

Galápagos Islands↗

USGS2020

Upwelling buffers climate change impacts on coral reefs of the eastern tropical Pacific

Corals of the eastern tropical Pacific live in a marginal and oceanographically dynamic environment. Along the Pacific coast of Panamá, stronger seasonal upwelling in the Gulf of Panamá in the east transitions to weaker upwelling in the Gulf of Chiriquí in the west, resulting in complex regional oceanographic conditions that drive differential coral-reef growth. Over millennial timescales, reefs in the Gulf of Chiriquí recovered more quickly from climatic disturbances compared with reefs in the Gulf of Panamá. In recent decades, corals in the Gulf of Chiriquí have also had higher growth rates than in the Gulf of Panamá. As the ocean continues to warm, however, conditions could shift to favor the growth of corals in the Gulf of Panamá, where upwelling may confer protection from high-temperature anomalies. Here we describe the recent spatial and temporal variability in surface oceanography of nearshore environments in Pacific Panamá and compare those conditions with the dynamics of contemporary coral-reef communities during and after the 2016 coral-bleaching event. Although both gulfs have warmed significantly over the last 150 yr, the annual thermal maximum in the Gulf of Chiriquí is increasing faster, and ocean temperatures there are becoming more variable than in the recent past. In contrast to historical trends, we found that coral cover, coral survival, and coral growth rates were all significantly higher in the Gulf of Panamá. Corals bleached extensively in the Gulf of Chiriquí following the 2015–2016 El Niño event, whereas upwelling in the Gulf of Panamá moderated the high temperatures caused by El Niño, allowing the corals largely to escape thermal stress. As the climate continues to warm, upwelling zones may offer a temporary and localized refuge from the thermal impacts of climate change, while reef growth in the rest of the eastern tropical Pacific continues to decline.

Gulf of Chiriqui, Gulf of Panama↗

USGS2020

Imprint of regional oceanography on foraminifera of eastern Pacific Coral Reefs

The marginal marine environments of the eastern tropical Pacific (ETP) serve as an ideal natural laboratory to study how oceanographic and climatic variability influence coral-reef ecosystems. Reefs along the Pacific coast of Panamá span a natural gradient of nutrients, pH, and temperature as a result of stronger seasonal upwelling in the Gulf of Panamá relative to the Gulf of Chiriquí. The ecosystems are not only influenced by spatial and seasonal variations in oceanography but are affected by the climatic variability of the El Niño-Southern Oscillation (ENSO). Foraminifera can be robust indicators of ecosystem condition because the composition of their assemblages and the geochemistry of their tests can change rapidly in response to environmental variability. We studied benthic foraminifera in sediment samples collected from 3 m below mean sea level in the Gulf of Panamá and the Gulf of Chiriquí. Temperature loggers deployed from 2016 to 2019 showed that average temperatures were lower and more variable in the Gulf of Panamá due to seasonal upwelling. All sites in both gulfs were dominated by heterotrophic foraminifera, which was likely the result of nutrient enrichment due to upwelling, combined with ENSO effects. However, the Gulf of Chiriquí was characterized by higher abundances of symbiont-bearing foraminifera than the Gulf of Panamá. The orders Miliolida and Rotaliida dominated the foraminiferal assemblages in both gulfs, with Quinqueloculina and Rosalina being the most abundant genera in the two orders, respectively. Miliolids were less abundant in the Gulf of Panamá than in the Gulf of Chiriquí, whereas rotaliid densities were not significantly different between the two gulfs. Lower pH in the Gulf of Panamá as a result of upwelling may have contributed to the lower abundance of miliolids, which secrete tests of high-magnesium calcite. Geochemical analysis of tests of the symbiont-bearing miliolid Sorites marginalis revealed that foraminiferal Mg/Ca ratios were lower in the Gulf of Panamá than in the Gulf of Chiriquí. The offset in foraminiferal Mg/Ca is consistent with the lower mean annual temperature observed in the Gulf of Panamá due to stronger seasonal upwelling. Because the geochemistry and assemblages of foraminifera reflect differences in environmental conditions, they could potentially be used in tandem with coral proxies to reconstruct past environmental change and project the future of coral-reef systems within the ETP.

Gulf of Chiriqui, Gulf of Panama↗

USGS2020

Digging into the geologic record of environmentally driven changes in coral-reef development

This lesson uses data based on real-world geological archives to guide students toward understanding how climate and oceanography have impacted coral-reef growth over the last 5000 years. The objective of the lesson is for students to determine the relationship between environmental variability and coral-reef development over millennial timescales. In this activity, students will: 1. Characterize the species composition and condition of coral reefs from different time periods in the past using cores of reef architecture 2. Calculate the rate of calcium carbonate accretion (production) of the reefs during those past time intervals 3. Reconstruct trends in past climatic conditions using a mock data-set.

Oceanography↗

USGS2019

Heat accumulation on coral reefs mitigated by internal waves

Coral reefs are among the most species-rich, productive and economically valuable ecosystems on Earth but increasingly frequent pantropical coral bleaching events are threatening their persistence on a global scale. The 2015–2016 El Niño led to the hottest sea surface temperatures on record and widespread bleaching of shallow-water corals. However, the causes of spatial variation in bleaching are poorly understood, and near-surface estimates of heat stress, such as those inferred from satellites, cannot be generalized across the broad depth ranges occupied by corals. Here, using in situ temperatures recorded across reefs from the near surface to 30–50 m depths in the western, central and eastern Pacific, we show that during the peak of the 2015–2016 anomaly, temperature fluctuations associated with internal waves reduced cumulative heat exposure by up to 88%. The durations of severe thermal anomalies above 8 °C-days, at which point widespread coral bleaching and mortality are likely, were also decreased by >36% at some sites and were prevented entirely at others. The impact of internal waves across depths on coral reefs has the potential to create and support thermal refuges in which heat stress and coral bleaching risk may be modulated, but future effects depend on the response of internal wave climates to continued warming and strengthening ocean stratification.

Nature Geoscience↗

USGS2019

Characterization of the exoskeleton of the Antarctic king crab Paralomis birsteini

Ocean acidification is projected to inhibit the biogenic production of calcium-carbonate skeletons in marine organisms. Antarctic waters represent a natural environment in which to examine the long-term effects of carbonate undersaturation on calcification in marine predators. King crabs (Decapoda: Anomura: Lithodidae), which currently inhabit the undersaturated environment of the continental slope off Antarctica, are potential invasives on the Antarctic shelf as oceanic temperatures rise. Here, we describe the chemical, physical, and mechanical properties of the exoskeleton of the deep-water Antarctic lithodid Paralomis birsteini and compare our measurements with two decapod species from shallow water at lower latitudes: Cancer borealis (Brachyura: Cancridae) and Callinectes sapidus (Brachyura: Portunidae). Paralomis birsteini deposit proportionally more calcium carbonate in their predatory chelae than their protective carapaces, compared with the other two crab species. When exoskeleton thickness and microhardness were compared between the chelae and carapace, the magnitude of the difference between these body regions was significantly greater in P. birsteini than in the other species tested. Hence, there appeared to be a greater disparity in P. birsteini in overall investment in calcium-carbonate structures among regions of the exoskeleton. The imperatives of prey consumption and predator avoidance may be influencing the deposition of calcium to different parts of the exoskeleton in lithodids living in an environment undersaturated with respect to calcium carbonate.

Invertebrate Biology↗

USGS2019

The 4.2 ka event, ENSO, and coral reef development

Variability of sea-surface temperature related to shifts in the mode of the El Niño–Southern Oscillation (ENSO) has been implicated as a possible forcing mechanism for the global-scale changes in tropical and subtropical precipitation known as the 4.2 ka event. We review records of coral reef development and paleoceanography from the tropical eastern Pacific (TEP) to evaluate the potential impact of the 4.2 ka event on coral reefs. Our goal is to identify the regional climatic and oceanographic drivers of a 2500-year shutdown of vertical reef accretion in the TEP after 4.2 ka. The 2500-year hiatus represents ∼40 % of the Holocene history of reefs in the TEP and appears to have been tied to increased variability of ENSO. When ENSO variability abated approximately 1.7–1.6 ka, coral populations recovered and vertical accretion of reef framework resumed apace. There is some evidence that the 4.2 ka event suppressed coral growth and reef accretion elsewhere in the Pacific Ocean as well. Although the ultimate causality behind the global 4.2 ka event remains elusive, correlations between shifts in ENSO variability and the impacts of the 4.2 ka event suggest that ENSO could have played a role in climatic changes at that time, at least in the tropical and subtropical Pacific. We outline a framework for testing hypotheses of where and under what conditions ENSO may be expected to have impacted coral reef environments around 4.2 ka. Although most studies of the 4.2 ka event have focused on terrestrial environments, we suggest that understanding the event in marine systems may prove to be the key to deciphering its ultimate cause.

Climate of the Past↗

USGS2018

How do upwelling and El Niño impact coral reef growth? A guided, inquiry-based lesson

This lesson uses real-world data to guide students toward understanding how climate and ocean variables impact coral reef growth. To begin this activity, students hypothesize how changes in environmental conditions could affect coral reef growth. They then compare metrics for reef growth (linear growth and percent coral cover) between two reefs in Pacific Panamá that are located in oceanographically and environmentally different embayments, or gulfs. A discussion following the first two activities allows the students to explore possible reasons for the observed differences between the reefs. Students then use their data to calculate a carbonate budget to estimate the rate of reef growth in each gulf. The purpose of calculating a carbonate budget is to provide students with an opportunity to estimate how variables such as coral growth rates, percent coral cover, and bioerosion contribute to the long-term potential for accretion or deterioration of coral reefs.

Oceanography↗

USGS2015

Holocene variability in the intensity of wind-gap upwelling in the tropical eastern Pacific

Wind-driven upwelling in Pacific Panamá is a significant source of oceanographic variability in the tropical eastern Pacific. This upwelling system provides a critical teleconnection between the Atlantic and tropical Pacific that may impact climate variability on a global scale. Despite its importance to oceanographic circulation, ecology, and climate, little is known about the long-term stability of the Panamanian upwelling system or its interaction with climatic forcing on millennial time scales. Using a combination of radiocarbon and U-series dating of fossil corals collected in cores from five sites across Pacific Panamá, we reconstructed the local radiocarbon reservoir correction, ΔR, from ~6750 cal B.P. to present. Because the ΔR of shallow-water environments is elevated by upwelling, our data set represents a millennial-scale record of spatial and temporal variability of the Panamanian upwelling system. The general oceanographic gradient from relatively strong upwelling in the Gulf of Panamá to weak-to-absent upwelling in the Gulf of Chiriquí was present throughout our record; however, the intensity of upwelling in the Gulf of Panamá varied significantly through time. Our reconstructions suggest that upwelling in the Gulf of Panamá is weak at present; however, the middle Holocene was characterized by periods of enhanced upwelling, with the most intense upwelling occurring just after of a regional shutdown in the development of reefs at ~4100 cal B.P. Comparisons with regional climate proxies suggest that, whereas the Intertropical Convergence Zone is the primary control on modern upwelling in Pacific Panamá, the El Niño–Southern Oscillation drove the millennial-scale variability of upwelling during the Holocene.

Gulf of Chiriqui, Gulf of Panama↗

USGS2003

Acropora corals in Florida: status, trends, conservation, and prospects for recovery

Despite representing the northern extent of Acropora spp. in the Caribbean, most of the Florida reef line from Palm Beach through the Keys was built by these species. Climatic factors appear to have bee important agents of Acropora loss within historic (century) time frames. In the recent past (1980-present), available quantitative evidence suggests dramatic declines occurred in A. cervicornis first (late 70's to 84) with collapse of A. palmata occuring later (1981-86). However, recent monitoring studies (1996-2001) show continued decline of remnant populations of A. palmata . Current trends in A. cervicornis in the Florida Keys are hard to assess given its exceedingly low abundance, except in Broward County, FL where recently discovered A. cervicornis thickets are thriving. While the State of Florida recognizes A. palmata and A. cervicornis as endangered species (Deyrup and Franz 1994), this designation carries no management implications. The current management plan of the FKNMS provides many strategies for coral conservation, among them minimizing the threat of vessel groundings and anchor damage, and prohibitions on collection, touching, and damage from fishery and recreational users. Although Acropopra spp. are not explicitly given any special consideration, they are implicitly by Santuary management. Restoration approaches undertaken in the Florida Keys include rescue of fragments damaged by groudings and experimental work to culture broadcast-spawned larvae to re-seed natural substrates. Neither of these efforts have yet realized full success.

Book↗