Edited by: Tamotsu Oomori, University of the Ryukyus, Japan
Reviewed by: Thomas M. DeCarlo, The University of Western Australia, Australia; Ryuji Asami, Tohoku University, Japan
†These authors have contributed equally to this work and share first authorship
This article was submitted to Coral Reef Research, a section of the journal Frontiers in Marine Science
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Reconstruction of sea surface temperature (SST) from the δ18O and Sr/Ca composition of coral skeletal density banding (CSDB), identified with x-ray diffraction and micro computed tomography, provides invaluable centuries-long records of ocean circulation and climate change. Comparison with age-equivalent instrument measurements of SST over the last 125 years has proven these δ18O-derived SST reconstructions to be generally reliable. However, notable exceptions occur within discrete CSDB stratigraphic intervals that yield δ18O-derived SST underestimates of as much as 9°C with respect to instrument measured SST. Here we combine high-resolution optical and electron microscopy with geochemical modeling to establish correction factors for the impact of marine seafloor physical, chemical, and biological alteration (
The record-breaking warm period of 2015 through 2018 continues the dramatic climb of average Earth temperatures over 17 of the last 18 years (
Chronostratigraphy provided by CSDB, as identified with x-ray diffraction (XRD) and micro computed tomography (MicroCT), is the primary contextual temporal framework used to reconstruct and globally correlate the timing and magnitude of past changes in SST (
Another especially critical factor is that coralla undergo early post-depositional physical, chemical and biological alteration in the marine seafloor environment in which they originally grew (
Our study, which directly builds upon previous research, is the first to fully contextualize and integrate high-resolution microscopy with mass balance binary mixing models to establish reliable and reproducible correction factors for the impact of seafloor diagenesis on δ18O-derived SST. Four cores are analyzed from
Coral skeletal cores were collected under the Australian Government Great Barrier Reef Marine Park Authority Permit G15/37167.1 issued to M.J. Kingsford. Four colonies of
Maximum intensity projection of microcomputed tomography (MicroCT) from 2 mm-thick equivalent slices (a total of 28 slices each having 69 μm isodiametric voxel size per slice) oriented vertically through the center of four
Intact cores were imaged at ∼60 μm-resolution on a custom-designed MicroCT system (North Scientific Instrument, Feinfocus 225 kV) in the University of Texas Computed Tomography (UTCT) high-resolution x-ray imaging facility. After acquiring the 3D data, each core was cut through the center into vertical 5 mm-thick and 2 mm-thick slices. The 5 mm-thick slices were also imaged using a standard XRD (Siemens Model #10092624, 70 kV) system at the Veterinary School of Medicine at the University of Illinois Urbana-Champaign. Further details are presented in the extended
From the central 5 mm-thick core slices, a total of 35 billets were cut, distributed over 2.1 m of the entire 2.3 m length of all four cores (representing 90% coverage). From each billet, an ultrathin 25 mm-thick ultra-thin section was prepared at Wagner Petrographic, Linden, Utah. Each thin section was impregnated with blue-dyed epoxy, doubly polished and mounted onto standard biological-sized glass slides. Further details are in the extended
A suite of integrated optical microscopy techniques were applied to determine the crystalline structure of the
Scanning electron microscopy was used to determine ultra-fine details of
HDB and LDB porosity was calculated from quantitative analyses of 420 high-resolution BF images collected at 250 nm-resolution from across all 35 thin sections in each of the four
The techniques used to process the 3D Micro CT images is presented above. All SEM images were presented without any intensity corrections. All optical microscopy images were processed in native Zeiss Zen Black (Airyscan) or Zen Blue (all other modalities) software. The entirety of raw data collected for each image presented in this manuscript is available upon request and has also been uploaded to the Figshare website as described in the manuscript. The images are adjusted after following the ethical procedures as described previously (
High-resolution microscopy was combined with binary mixing modeling to quantitatively estimate the impact of seafloor diagenesis on sea surface temperature (SST) paleothermometry reconstructions. A binary mixing line was calculated from
The BF images from bottom 6 cm of HDBs at multiple locations in the base of Cores 3 and 4 were used for extracting a range of cement and primary skeleton mixing ratios. Using Core 3 as an example, a red circle with an area of 1 mm was drawn to represent the typical 1 mm-diameter drill tip. Then the images were thresholded based on the RGB values of the blue color epoxy used for the thin sectioning using the 2D image analysis program Axiovision 4.93 (Carl Zeiss, Oberkochen, Germany). The thresholded areas are restricted to inside the 1 mm drill trip by tracing the red line and areas of blue pixels were measured in pixel2. Then using the same program, areas of the occlusion (cement) together with remaining porosity (blue areas) were traced based on the sclerodermite transition region as a marker. Inside the traces the area in pixel2 is determined, leaving the primary skeleton. This area is subtracted from the 1 mm drill tip area to assess the primary skeleton area. To calculate the cement area, the blue epoxy area is subtracted from the tracings of sclerodermite traced area, then the percent cement is calculated by using the 1 mm drill trip area over the cement area. Based on these, the absolute skeleton area, cement area and remaining porosity area have been calculated and adding their percent lead up to 100%. The cement area is set at “0” and primary skeleton area set at 100% for the ∼0% location of the HDB. All locations are retrieved from HDB areas at the bottom of Core 3. This data is presented in
Coral skeletal density banding stratigraphic sequences are cross-sections of corallum morphology that reflect meter-scale whole head hemispherical curvature, centimeter-scale surface topography and millimeter-scale shapes of individual polyp skeletal cups (
The top few millimeters of each core is the zero-age accreting surface where living coral tissue is actively secreting pristine skeleton. Corallite skeletal walls in these uppermost surface layers can therefore be considered original crystallization products that result from combined coral tissue cellular controls and environmental influences (
Quantitative analyses of brightfield (BF) high-resolution images (
Crystalline structure of high- and low-density bands (HDBs and LDBs) within the uppermost 2 cm of Core 3.
Crystalline structure of high- and low-density bands (HDBs and LDBs) within the lowermost 6 cm of Core 3.
Ring aperture contrast images of crystalline structure and diagenesis in high- and low-density bands (HDBs and LDBs) within the uppermost (top 6 cm,
Uppermost growing surface at the top of Core 3 showing finger-like skeletal growth projections.
Within the original coral skeletal walls, multiple 5–20 μm-diameter clusters are observed composed of microcrystalline aragonite (
Examples of brightfield (BF;
Brightfield (BF;
Previous studies have described sclerodermites as arrays of single large orthorhombic crystals of aragonite with unit extinction under polarization (called “fibers”) (
Skeletal walls are composed of multiple sclerodermites that grow toward and eventually abut against one another (
Evidence from super-resolution, optical and electron microscopy proves that more than 95% of the total skeletal volume comprising the four cores exhibit little to no diagenetic alteration. In stark contrast, extensive diagenetic aragonite cementation is concentrated within specific CSDB intervals near the base of core 3 at 24 m WD and core 4 at 14 m WD (
High density bands below the uppermost 6 cm in each core have an average porosity of 30% (±7%;
Scanning electron microscope (SEM) images showing extensive diagenetic aragonite cements (AC) occluding pore space within a high-density band (HDB) from the bottom 6 cm of Core 3.
Brightfield (BF) and polarization (POL) images showing extensive diagenetic aragonite cement occluding high- and low-density band (HDB and LDB) pore space in the bottom 6 cm of Core 3.
In addition, diagenetic fronts are common that migrate through and solubilize (
Detailed comparative analyses of all four
High-resolution microscopy indicates that coral SST paleothermometry is predominantly impacted by diagenetic aragonite cementation in skeletal pores (
However, microscopy in the present study indicates that seafloor diagenetic alteration of CSDB has the capacity to markedly alter the thickness, intensity, lateral continuity and mineralogical and geochemical character of these CSDB sequences (
The growth of aragonite cements similar to those within
Example integration of quantitative petrography with binary mixing modeling to estimate correction factors for the impact of seafloor diagenesis on δ18O-derived SST reconstructs from coral skeletons.
The majority of SST reconstructions from coral skeletons are made from sample powders collected using 1 mm-diameter drill tips [e.g.,
Multiple lines of evidence indicate that this integration of quantitative high-resolution petrography with binary mixing modeling is required in future studies to establish correction factors for δ18O-derived SST (
Multiple lines of evidence indicate that the integration of high-resolution microscopy with binary mixing models (
In conclusion, marine carbonate research has shown that variable extents of seafloor diagenetic aragonite cementation (ranging from being absent and rare to common and abundant) possibly takes place within coral skeletons around the world (
This paper, which directly builds upon previous research, is the first to fully contextualize and integrate high-resolution microscopy with mass balance binary mixing models to establish reliable and reproducible correction factors for the impact of seafloor diagenesis on δ18O-derived SST reconstructions. Four cores are analyzed from
The raw data supporting the conclusions of this manuscript will be made available by the authors, without undue reservation, to any qualified researcher.
MK, KyF, and BF collected the coral cores from Myrmidon Reef. MS, KyF, LT, and BF performed the experiments, analyzed the data, and prepared final images. MS collected and interpreted MicroCT data. MS, KyF, and BF drafted the manuscript. BF supervised the project. MS, KyF, LT, MK, KaF, JT, and BF conducted collaborative research support throughout the study, reviewed and edited the entire manuscript continuously leading up to the submission version.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We sincerely thank M. O’Callaghan, K. Hartog-Burnett and other members of the Kingsford laboratory at James Cook University for their tireless teamwork in collecting the coral cores on Myrmidon Reef. We also thank G. A. Fried for expertise in preparing and cutting coral cores, E. Fried for assistance with preparation and analysis of the coral cores at the University of Texas Computed Tomography (UTCT) laboratory, and M. W. Colbert and J. Maisano for their extraordinary efforts and expertise in completing the MicroCT analyses at UTCT. K. Lee conducted EDXS analyses at the University of Illinois Urbana–Champaign Beckman Institute. We also thank J. Conroy for advice and suggestions regarding paleothermometry and climate predictions. We are also appreciative to L. Z. DeVille for in-depth discussion on mixing curves and his expertise in mathematic modeling and L. Trevor, Application Engineer, Materials and Structural Analysis Division (MSD) at ThermoFisher Scientific for help with creating the 3-D rendering of corals from MicroCT data using Amira software. We are also thankful to Aaron King at the Department of Geography and Geographic Information Science, University of Illinois at Urbana–Champaign for helping us create high resolution Australia sample location maps using ArcGIS software.
The Supplementary Material for this article can be found online at: