Browsing by Author "Carilli, JE"
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- ItemBenthic foraminiferal assemblages from Kiritimati (Christmas) Island indicate human-mediated nutrification has occurred over the scale of decades(Inter-Research Science Publisher, 2012-06-07) Carilli, JE; Walsh, SCommunity assemblages of live and dead benthic foraminifera from Kiritimati (Christmas) Island, Kiribati, were used to investigate changes in nutrification before and after human occupation. Benthic foraminiferal assemblages have previously been shown to have strong empirical relationships with water quality: mixotrophic, symbiont-bearing foraminifera dominate in clear, nutrient-poor waters, while heterotrophic and/or opportunistic foraminifera are more prevalent in polluted or nutrified waters. After human occupation, the proportion of mixotrophic taxa decreased significantly at all sites on Kiritimati with the largest decreases observed at sites with the highest fishing pressure. These changes in the benthic foraminiferal assemblage indicate that nutrification has occurred on Kiritimati over the scale of decades, possibly due to changes in trophic structure and nutrient cycling caused by fishing. © 2012, Inter-Research.
- ItemBomb fall-out 236U as a global oceanic tracer using an annually resolved coral core(Elsevier, 2012-12-15) Winkler, SR; Steler, P; Carilli, JEAnthropogenic 236U (t½=23.4 My) is an emerging isotopic ocean tracer with interesting oceanographic properties, but only with recent advances in accelerator mass spectrometry techniques is it now possible to detect the levels from global fall-out of nuclear weapons testing across the water column. To make full use of this tracer, an assessment of its input into the ocean over the past decades is required. We captured the bomb-pulse of 236U in an annually resolved coral core record from the Caribbean Sea. We thereby establish a concept which gives 236U great advantage – the presence of reliable, well-resolved chronological archives. This allows studies of not only the present distribution pattern, but gives access to the temporal evolution of 236U in ocean waters over the past decades. © 2012, Elsevier Ltd.
- ItemCentury-scale records of coral growth rates indicate that local stressors reduce coral thermal tolerance threshold(Wiley-Blackwell Publishing, 2010-04-01) Carilli, JE; Norris, RD; Black, B; Walsh, S; McField, MCoral bleaching, during which corals lose their symbiotic dinoflagellates, appears to be increasing in frequency and geographic extent, and is typically associated with abnormally high water temperatures and solar irradiance. A key question in coral reef ecology is whether local stressors reduce the coral thermal tolerance threshold, leading to increased bleaching incidence. Using tree-ring techniques, we produced master chronologies of growth rates in the dominant reef builder, massive Montastraea faveolata corals, over the past 75-150 years from the Mesoamerican Reef. Our records indicate that the 1998 mass bleaching event was unprecedented in the past century, despite evidence that water temperatures and solar irradiance in the region were as high or higher mid-century than in more recent decades. We tested the influence on coral extension rate from the interactive effects of human populations and thermal stress, calculated here with degree-heating-months (DHM). We find that when the effects of chronic local stressors, represented by human population, are taken into account, recent reductions in extension rate are better explained than when DHM is used as the sole predictor. Therefore, the occurrence of mass bleaching on the Mesoamerican reef in 1998 appears to stem from reduced thermal tolerance due to the synergistic impacts of chronic local stressors. © 2010, Wiley-Blackwell.
- ItemEquatorial Pacific coral geochemical records show recent weakening of the Walker Circulation(American Geophysical Union, 2014-11-10) Carilli, JE; McGregor, HV; Gaudry, JJ; Donner, SD; Gagan, MK; Stevenson, S; Wong, HKY; Fink, DEquatorial Pacific ocean-atmosphere interactions affect climate globally, and a key component of the coupled system is the Walker Circulation, which is driven by sea surface temperature (SST) gradients across the equatorial Pacific. There is conflicting evidence as to whether the SST gradient and Walker Circulation have strengthened or weakened over the late twentieth century. We present new records of SST and sea surface salinity (SSS) spanning 1959–2010 based on paired measurements of Sr/Ca and δ18O in a massive Porites coral from Butaritari atoll in the Gilbert Islands, Republic of Kiribati, in the central western equatorial Pacific. The records show 2–7 year variability correlated with the El Niño–Southern Oscillation (ENSO) and corresponding shifts in the extent of the Indo-Pacific Warm Pool, and decadal-scale signals related to the Pacific Decadal Oscillation and the Pacific Warm Pool Index. In addition, the Butaritari coral records reveal a small but significant increase in SST (0.39°C) from 1959 to 2010 with no accompanying change in SSS, a trend that persists even when ENSO variability is removed. In contrast, larger increases in SST and SSS are evident in coral records from the equatorial Pacific Line Islands, located east of Butaritari. Taken together, the equatorial Pacific coral records suggest an overall reduction in the east-west SST and SSS gradient over the last several decades, and a recent weakening of the Walker Circulation. © 2014, American Geophysical Union. All Rights Reserved.
- ItemHistorical temperature variability affects coral response to heat stress(Public Libray of Science, 2012-03-30) Carilli, JE; Donner, SD; Hartmann, ACCoral bleaching is the breakdown of symbiosis between coral animal hosts and their dinoflagellate algae symbionts in response to environmental stress. On large spatial scales, heat stress is the most common factor causing bleaching, which is predicted to increase in frequency and severity as the climate warms. There is evidence that the temperature threshold at which bleaching occurs varies with local environmental conditions and background climate conditions. We investigated the influence of past temperature variability on coral susceptibility to bleaching, using the natural gradient in peak temperature variability in the Gilbert Islands, Republic of Kiribati. The spatial pattern in skeletal growth rates and partial mortality scars found in massive Porites sp. across the central and northern islands suggests that corals subject to larger year-to-year fluctuations in maximum ocean temperature were more resistant to a 2004 warm-water event. In addition, a subsequent 2009 warm event had a disproportionately larger impact on those corals from the island with lower historical heat stress, as indicated by lower concentrations of triacylglycerol, a lipid utilized for energy, as well as thinner tissue in those corals. This study indicates that coral reefs in locations with more frequent warm events may be more resilient to future warming, and protection measures may be more effective in these regions. © 2012 Carilli et al.
- ItemPeriodic endolithic algal blooms in Montastraea faveolata corals may represent periods of low-level stress.(Ingenta, 2010-07-01) Carilli, JE; Godfrey, J; Norris, RN; Sandin, SA; Smith, JECores from the scleractinian coral Montastraea faveolata (Ellis and Solander, 1786) sensu Weil and Knowlton (1994) from the Mesoamerican Reef possess obvious green bands, sometimes occurring annually, but more often at less frequent intervals. Bands are remnant concentrations of the endolithic green alga Ostreobium spp. that grow parallel to and below the living coral tissue. We dated green bands in 58 M. faveolata cores collected from four sites on the Mesoamerican Reef. We found that the bands are not related to coral skeletal growth reductions, but instead are caused by algal blooms within the coral skeleton. We hypothesize that the blooms occur during periods of coral paling (the partial loss of pigmentation), during which more light penetrates through the translucent coral tissue into the coral skeleton. This hypothesis is supported by observations of discontinuous banding within the skeleton, the patchiness of pigment loss in living corals, and ecological observations of endolithic algal blooms in living bleached corals. At three sites, there was a significant increase in green band occurrence over time, which suggests that coral paling may have increased over the last several decades. © 2020 Ingenta
- ItemRadiocarbon surface ocean reservoir ages over the past 6,000 years from Porites microatolls, Christmas (Kiritimati) Island, central Pacific Ocean(Australian Geosciences Council, 2012-08-05) Fink, D; Carilli, JE; McGregor, HV; Woodroffe, CD; Zhao, JX; Fallon, SJWe present radiocarbon reservoir (ΔR) values over the past 5,000 years based on high-precision paired U-series and AMS 14C in modern and fossil Porites coral micro-atolls from Christmas Island (2N, 157W). The data set (n∼25) allows temporal reconstruction of ΔR with ∼250 year spacing. Christmas Island lies within the Equatorial Counter Current and the NINO3.4 region with a climate frequently punctuated by higher precipitation and warmer SSTs during El-Nino. Along its coastal perimeter, and throughout the internal network of tidal flats and lagoons, which in the late Holocene were flourishing reefal environments, fossil and modern microatolls abound. Microatolls are large discoid colonies with horizontal radial growth axes constrained in upwards vertical growth by spring tide low-water level. Consequently, a single microatoll can extend to ∼9 m in diameter representing an exceptional ∼300 years of continuous growth. Sr/Ca and δ18O in modern microatolls faithfully replicate instrumental climate records (SSTs, rainfall). Results from numerous fossil cores (1500–5500 years BP) shows distinct variability in ENSO variance both in frequency and amplitude domains compared to modern microatolls. These fossil populations are used to generate a temporal pattern of ΔR variability. Strict criteria were followed with respect to secondary calcite (via quantitative XRD), unaltered microstructure (via thin sections) and ensuring splits of identical coral chips for dual U-series and AMS 14C. Preliminary results show a ΔR decrease at about 2000 BP. Further analyses of correlations of ΔR deviations from the long term average with changes in regional ocean surface currents and in paleo-ENSO variance are in progress.
- ItemReply to comment by Karnauskas et al. on “Equatorial Pacific coral geochemical records show recent weakening of the Walker circulation”(American Geophysical Union, 2015-05-18) Carilli, JE; McGregor, HV; Gaudry, JJ; Donner, SD; Gagan, MK; Stevenson, S; Wong, HKY; Fink, DIn our paper describing a new coral record from Butaritari, we hypothesized that comparing the temporal trends in our records to coral records from farther east in the equatorial Pacific may support the evidence for a weakening of a Walker circulation, documented elsewhere in the literature [Power and Smith, 2007; Tokinaga et al., 2012]. Weakening of the Walker circulation is expected under global warming due to an imbalance in the rate of change in different aspects of the hydrological cycle [Vecchi and Soden, 2007]. We thank Karnauskas et al. [2015] for recognizing the value of our Butaritari coral climate reconstruction, and we appreciate their critique of our study. The Karnauskas et al. [2015] analyses strengthen our argument regarding the utility of interisland coral-proxy derived sea surface temperature (SST) gradients as a Walker circulation metric, but we disagree with their interpretation of decadal variability in our records. Here we provide additional analyses, which confirm that our reconstruction [Carilli et al., 2014] shows a long-term weakening of the Walker circulation over 1972–1998. We also document that significant decadal variations in Walker circulation strength, and for particular choices of start and end years over which trends are calculated, are able to show slight Walker strengthening. Overall, we conclude that Walker circulation variations are more nuanced than either our original publication [Carilli et al., 2014] or the subsequent Karnauskas et al. [2015] comment would suggest. Karnauskas et al. [2015] also provide a detailed analysis of Equatorial Undercurrent (EUC) activity near the Gilbert Islands and argue that the EUC does not strongly affect Butaritari. Our original publication did not claim to find significant EUC/Butaritari linkages, and we appreciate the diligence of Karnauskas et al. [2015] for ruling this out as a possibility. © 2015, American Geophysical Union.