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Upwellings mitigated Plio-Pleistocene heat stress for reef corals on the Florida platform (USA)
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|Authors:||Brachert, Thomas C.; Reuter, Markus; Krüger, Stefan; Kirkerowicz, Julia; Klaus, James S.|
|Author Affiliations:||Primary: |
Universität Leipzig, Institut für Geophysik und Geologie, Leipzig, Germany
Karl Franzens Universität Graz, Austria
University of Miami, United States
|Source:||Biogeosciences, 13(5), p.1469-1489. Publisher: Copernicus GmbH on behalf of the European Union, Katlenburg-Lindau, International. ISSN: 1726-4170|
|Note:||In English. 77 refs.; illus., incl. 3 tables, sketch map|
|Summary:||The fast growing calcareous skeletons of zooxanthellate reef corals (z corals) represent unique environmental proxy archives through their oxygen and carbon stable isotope composition (δ18O, δ13C). In addition, the accretion of the skeleton itself is ultimately linked to the environment and responds with variable growth rates (extension rate) and density to environmental changes. Here we present classical proxy data (δ18O, δ13C) in combination with calcification records from 15 massive z corals. The z corals were sampled from four interglacial units of the Florida carbonate platform (USA) dated approximately 3.2, 2.9, 1.8 and 1.2 Ma (middle Pliocene to early Pleistocene). The z corals (Solenastrea, Orbicella, Porites) derive from unlithified shallow marine carbonates and were carefully screened for primary preservation suited for proxy analysis. We show that skeletal accretion responded with decreasing overall calcification rates (decreasing extension rate but increasing density) to warmer water temperatures. Under high annual water temperatures, inferred from sub-annually resolved δ18O data, skeletal bulk density was high, but extension rates and overall calcification rates were at a minimum (endmember scenario 1). Maximum skeletal density was reached during the summer season giving rise to a growth band of high density within the annually banded skeletons ("high density band", HDB). With low mean annual water temperatures (endmember scenario 2), bulk skeletal density was low but extension rates and calcification rates reached a maximum, and under these conditions the HDB formed during winter. Although surface water temperatures in the Western Atlantic warm pool during the interglacials of the late Neogene were ∼ 2 °C higher than they are in the present day, intermittent upwelling of cool, nutrient-rich water mitigated water temperatures off south-western Florida and created temporary refuges for z coral growth. Based on the sub-annually resolved δ18O and δ13C records, the duration of the upwelling episodes causing the endmember 2 conditions was variable and lasted from a few years to a number of decades. The episodes of upwelling were interrupted by phases without upwelling (endmember 1) which lasted for at least a few years and led to high surface water temperatures. This variable environment is likely one of the reasons why the coral fauna is dominated by the eurytopic genus Solenastrea, also a genus resistant to high turbidity. Over a period of ∼ 50 years, the oldest sub annually resolved proxy record available (3.2 Ma) documents a persistent occurrence of the HDB during winter. In contrast, the HDB forms in summer in modern z corals from the Florida reef tract. We suggest this difference should be tested as being the expression of a tendency towards decreasing interglacial upwelling since the middle Pliocene. The number of z coral sclerochronological records for the Plio-Pleistocene is still rather low, however, and requires more data and an improved resolution, through records from additional time slices. Nonetheless, our calcification data from the warm periods of past interglacials may contribute to predicting the effects of future ocean warming on z coral health along the Florida reef tract. The inconsistent timing of the HDB within single coral records or among specimens and time slices is unexpected and contrasts the common practice of establishing chronologies on the basis of the density banding.|
|Subjects:||Annual variations; Anthozoa; Bulk density; C-13/C-12; Calcification; Caloosahatchee Formation; Carbon; Carbonate platforms; Carbonate sediments; Cenozoic; Chemical composition; Cnidaria; Cycles; Electron microscopy data; Exoskeletons; Growth rates; Indicators; Interglacial environment; Isotope ratios; Isotopes; Lower Pleistocene; Marine environment; Middle Pliocene; Neogene; Nutrients; O-18/O-16; Ocean circulation; Oxygen; Paleo-oceanography; Paleoenvironment; Paleotemperature; Pleistocene; Pliocene; Preservation; Productivity; Quaternary; Reef environment; Sea water; Sea-surface temperature; Seasonal variations; Sediments; SEM data; Stable isotopes; Tamiami Formation; Tertiary; Upper Cenozoic; Upwelling; Zooxanthellae; Florida; United States; Bee Branch Member; Bermont Formation; Florida Platform; Golden Gate Member; Holey Land Member; Solenastrea|
|Coordinates:||N260000 N280000 W0800000 W0820000|
|Copyright Information:||GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Copernicus Gesellschaft, Katlenburg-Lindau, Germany|
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