Mollica Nathaniel R.

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Last Name
Mollica
First Name
Nathaniel R.
ORCID
0000-0003-4751-8061

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Now showing 1 - 8 of 8
  • Article
    Mid-Holocene, coral-based sea surface temperatures in the western tropical Atlantic
    (American Geophysical Union, 2019-05-31) Rodriguez, Luis G. ; Cohen, Anne L. ; Ramirez, Wilson ; Oppo, Delia W. ; Pourmand, Ali ; Edwards, R. Lawrence ; Alpert, Alice ; Mollica, Nathaniel R.
    The Holocene is considered a period of relative climatic stability, but significant proxy data‐model discrepancies exist that preclude consensus regarding the postglacial global temperature trajectory. In particular, a mid‐Holocene Climatic Optimum, ~9,000 to ~5,000 years BP, is evident in Northern Hemisphere marine sediment records, but its absence from model simulations raises key questions about the ability of the models to accurately simulate climate and seasonal biases that may be present in the proxy records. Here we present new mid‐Holocene sea surface temperature (SST) data from the western tropical Atlantic, where twentieth‐century temperature variability and amplitude of warming track the twentieth‐century global ocean. Using a new coral thermometer Sr‐U, we first developed a temporal Sr‐U SST calibration from three modern Atlantic corals and validated the calibration against Sr‐U time series from a fourth modern coral. Two fossil corals from the Enriquillo Valley, Dominican Republic, were screened for diagenesis, U‐series dated to 5,199 ± 26 and 6,427 ± 81 years BP, respectively, and analyzed for Sr/Ca and U/Ca, generating two annually resolved Sr‐U SST records, 27 and 17 years long, respectively. Average SSTs from both corals were significantly cooler than in early instrumental (1870–1920) and late instrumental (1965–2016) periods at this site, by ~0.5 and ~0.75 °C, respectively, a result inconsistent with the extended mid‐Holocene warm period inferred from sediment records. A more complete sampling of Atlantic Holocene corals can resolve this issue with confidence and address questions related to multidecadal and longer‐term variability in Holocene Atlantic climate.
  • Article
    The challenges of detecting and attributing ocean acidification impacts on marine ecosystems
    (Oxford University Press, 2020-08-09) Doo, Steve ; Kealoha, Andrea K. ; Andersson, Andreas ; Cohen, Anne L. ; Hicks, Tacey L. ; Johnson, Zackary I. ; Long, Matthew H. ; McElhany, Paul ; Mollica, Nathaniel R. ; Shamberger, Kathryn E. F. ; Silbiger, Nyssa J. ; Takeshita, Yuichiro ; Busch, D. Shallin
    A substantial body of research now exists demonstrating sensitivities of marine organisms to ocean acidification (OA) in laboratory settings. However, corresponding in situ observations of marine species or ecosystem changes that can be unequivocally attributed to anthropogenic OA are limited. Challenges remain in detecting and attributing OA effects in nature, in part because multiple environmental changes are co-occurring with OA, all of which have the potential to influence marine ecosystem responses. Furthermore, the change in ocean pH since the industrial revolution is small relative to the natural variability within many systems, making it difficult to detect, and in some cases, has yet to cross physiological thresholds. The small number of studies that clearly document OA impacts in nature cannot be interpreted as a lack of larger-scale attributable impacts at the present time or in the future but highlights the need for innovative research approaches and analyses. We summarize the general findings in four relatively well-studied marine groups (seagrasses, pteropods, oysters, and coral reefs) and integrate overarching themes to highlight the challenges involved in detecting and attributing the effects of OA in natural environments. We then discuss four potential strategies to better evaluate and attribute OA impacts on species and ecosystems. First, we highlight the need for work quantifying the anthropogenic input of CO2 in coastal and open-ocean waters to understand how this increase in CO2 interacts with other physical and chemical factors to drive organismal conditions. Second, understanding OA-induced changes in population-level demography, potentially increased sensitivities in certain life stages, and how these effects scale to ecosystem-level processes (e.g. community metabolism) will improve our ability to attribute impacts to OA among co-varying parameters. Third, there is a great need to understand the potential modulation of OA impacts through the interplay of ecology and evolution (eco–evo dynamics). Lastly, further research efforts designed to detect, quantify, and project the effects of OA on marine organisms and ecosystems utilizing a comparative approach with long-term data sets will also provide critical information for informing the management of marine ecosystems.
  • Thesis
    Coral reefs in the Anthropocene Ocean: novel insights from skeletal proxies of climate change, impacts, and resilience
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2021-02) Mollica, Nathaniel R. ; Cohen, Anne L. ; Guo, Weifu
    Anthropogenic emissions of greenhouse gases are driving rapid changes in ocean conditions. Shallow-water coral reefs are experiencing the brunt of these changes, including intensifying marine heatwaves (MHWs) and rapid ocean acidification (OA). Consequently, coral reefs are in broad-scale decline, threatening the livelihoods of hundreds of millions of people. Ensuring survival of coral reefs in the 21st century will thus require a new management approach that incorporates robust understanding of reef-scale climate change, the mechanisms by which these changes impact corals, and their potential for adaptation. In this thesis, I extract information from within coral skeletons to 1) Quantify the climate changes occurring on coral reefs and the effects on coral growth, 2) Identify differences in the sensitivity of coral reefs to these changes, and 3) Evaluate the adaptation potential of the keystone reef-building coral, Porites. First, I develop a mechanistic Porites growth model and reveal the physicochemical link between OA and skeletal formation. I show that the thickening (densification) of coral skeletal framework is most vulnerable to OA and that, under 21st century climate model projections, OA will reduce Porites skeletal density globally, with greatest impact in the Coral Triangle. Second, I develop an improved metric of thermal stress, and use a skeletal bleaching proxy to quantify coral responses to intensifying heatwaves in the central equatorial Pacific (CEP) since 1982. My work reveals a long history of bleaching in the CEP, and reef-specific differences in thermal tolerance linked to past heatwave exposure implying that, over time, reef communities have adapted to tolerate their unique thermal regimes. Third, I refine the Sr-U paleo-thermometer to enable monthly-resolved sea surface temperatures (SST) generation using laser ablation ICPMS. I show that laser Sr-U accurately captures CEP SST, including the frequency and amplitude of MHWs. Finally, I apply laser Sr-U to reconstruct the past 100 years of SST at Jarvis Island in the CEP, and evaluate my proxy record of bleaching severity in this context. I determine that Porites coral populations on Jarvis Island have not yet adapted to the pace of anthropogenic climate change.
  • Article
    Increasing coral reef resilience through successive marine heatwaves
    (American Geophysical Union, 2021-08-30) Fox, Michael D. ; Cohen, Anne L. ; Rotjan, Randi ; Mangubhai, Sangeeta ; Sandin, Stuart A. ; Smith, Jennifer E. ; Thorrold, Simon R. ; Dissly, Laura ; Mollica, Nathaniel R. ; Obura, David
    Ocean warming is causing declines of coral reefs globally, raising critical questions about the potential for corals to adapt. In the central equatorial Pacific, reefs persisting through recurrent El Niño heatwaves hold important clues. Using an 18-year record of coral cover spanning three major bleaching events, we show that the impact of thermal stress on coral mortality within the Phoenix Islands Protected Area (PIPA) has lessened over time. Disproportionate survival of extreme thermal stress during the 2009–2010 and 2015–2016 heatwaves, relative to that in 2002–2003, suggests that selective mortality through successive heatwaves may help shape coral community responses to future warming. Identifying and facilitating the conditions under which coral survival and recovery can keep pace with rates of warming are essential first steps toward successful stewardship of coral reefs under 21st century climate change.
  • Article
    Repeat bleaching of a central Pacific coral reef over the past six decades (1960–2016)
    (Nature Publishing Group, 2018-11-08) Barkley, Hannah C. ; Cohen, Anne L. ; Mollica, Nathaniel R. ; Brainard, Russell E. ; Rivera, Hanny E. ; DeCarlo, Thomas M. ; Lohmann, George P. ; Drenkard, Elizabeth J. ; Alpert, Alice ; Young, Charles W. ; Vargas-Ángel, Bernardo ; Lino, Kevin C. ; Oliver, Thomas A. ; Pietro, Kathryn R. ; Luu, Victoria
    The oceans are warming and coral reefs are bleaching with increased frequency and severity, fueling concerns for their survival through this century. Yet in the central equatorial Pacific, some of the world’s most productive reefs regularly experience extreme heat associated with El Niño. Here we use skeletal signatures preserved in long-lived corals on Jarvis Island to evaluate the coral community response to multiple successive heatwaves since 1960. By tracking skeletal stress band formation through the 2015-16 El Nino, which killed 95% of Jarvis corals, we validate their utility as proxies of bleaching severity and show that 2015-16 was not the first catastrophic bleaching event on Jarvis. Since 1960, eight severe (>30% bleaching) and two moderate (<30% bleaching) events occurred, each coinciding with El Niño. While the frequency and severity of bleaching on Jarvis did not increase over this time period, 2015–16 was unprecedented in magnitude. The trajectory of recovery of this historically resilient ecosystem will provide critical insights into the potential for coral reef resilience in a warming world.
  • Article
    Ocean acidification has impacted coral growth on the great barrier reef
    (American Geophysical Union, 2020-08-27) Guo, Weifu ; Bokade, Rohit ; Cohen, Anne L. ; Mollica, Nathaniel R. ; Leung, Muriel ; Brainard, Russell E.
    Ocean acidification (OA) reduces the concentration of seawater carbonate ions that stony corals need to produce their calcium carbonate skeletons and is considered a significant threat to the functional integrity of coral reef ecosystems. However, detection and attribution of OA impact on corals in nature are confounded by concurrent environmental changes, including ocean warming. Here we use a numerical model to isolate the effects of OA and temperature and show that OA alone has caused 13 ± 3% decline in the skeletal density of massive Porites corals on the Great Barrier Reef since 1950. This OA‐induced thinning of coral skeletons, also evident in Porites from the South China Sea but not in the central Pacific, reflects enhanced acidification of reef water relative to the surrounding open ocean. Our finding reinforces concerns that even corals that might survive multiple heatwaves are structurally weakened and increasingly vulnerable to the compounding effects of climate change.
  • Article
    Capturing equatorial Pacific variability with multivariate Sr‐U coral thermometry
    (American Geophysical Union, 2023-09-27) Mollica, Nathaniel Rust ; Cohen, Anne L. ; Horton, Forrest ; Oppo, Delia W. ; Solow, Andrew R. ; McGee, David
    Sr-U, a coral-based paleothermometer, corrects for the effects of Rayleigh Fractionation on Sr/Ca by regressing multiple, paired U/Ca and Sr/Ca values. Prior applications of Sr-U captured mean annual sea surface temperatures (SSTs), inter-annual variability, and long-term trends. However, because many Sr/Ca-U/Ca pairs are needed for a single Sr-U value as originally formulated, the temporal resolution of the proxy is typically limited to 1 year. Here, we address this limitation by applying laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to three Porites colonies from Jarvis and Nikumaroro Islands in the central equatorial Pacific (CEP), generating ∼25 Sr/Ca-U/Ca pairs per month of skeletal growth. Both Sr/Ca and U/Ca vary significantly over small (sub-mm) length scales and support the calculation of Sr-U values using the original regression method. Over the represented temperature range of 24–31°C, the Sr/Ca-U/Ca-SST relationships are nonlinear, a finding consistent with predictions of the Rayleigh model. To reflect this non-linearity, we developed a calibration using multivariate nonlinear regression. The multivariate, three-coral calibration was applied to 20 years of monthly resolved Sr/Ca and U/Ca of a coral interval not included in the calibration, yielding RMSE = 0.73°C and r2 = 0.85 (p < 0.05; df = 256). The multivariate calibration performed significantly better than Sr/Ca alone (r2 = 0.28). Applying the new calibration to a subfossil Porites from Kiritimati Atoll, CEP (2200 Before Present) yields equivalent phase and amplitude of interannual variability, but water temperatures ∼1.6°C cooler than they are in this region today.
  • Article
    Coral Sr-U Thermometry tracks ocean temperature and reconciles Sr/Ca discrepancies caused by Rayleigh Fractionation
    (American Geophysical Union, 2023-07-14) Galochkina, Mariya ; Cohen, Anne L. ; Oppo, Delia W. ; Mollica, Nathaniel Rust ; Horton, Forrest
    Understanding climate change at the spatiotemporal scales necessary to improve climate projections requires proxy records that complement sparse and often contradictory observational temperature data sets. Massive long-lived corals have tremendous potential in this regard, continuously recording information about ocean conditions as they grow. Nevertheless, extracting accurate ocean temperatures from corals is challenging because factors other than temperature influence skeletal chemistry. Here, we tested the ability of the coral Sr-U thermometer to accurately capture annual sea surface temperatures (SSTs) in the subtropical Atlantic, where year-to-year temperatures vary by ∼1°C. Using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we generated sufficient U/Ca – Sr/Ca pairs from a slow-growing (1−2 mm/yr) Siderastrea siderea coral to calculate annual Sr-U values. With the fine-scale spatial resolution attained using the laser, skeleton accreted during both fast and slow growing times of the year was represented in our sampling. The resulting 30-year-long Sr-U record tracked the amplitude and timing of annual SST to within ±0.2°C of observations (r = −0.71), whereas the Sr/Ca record did not (r = 0.23). Furthermore, Sr-U corrected for Sr/Ca offsets among adjacent skeletal elements approximately 1 mm apart. These offsets are equivalent to differences of 2–3°C if typical Sr/Ca–SST calibrations are applied. Our observations indicate that Sr-U can accurately constrain decadal-to-multidecadal variability and secular SST trends in regions where this information is urgently needed.