The record of Pliocene sea-level change at Enewetak Atoll

doi:10.1016/0277-3791(91)90023-N

Quaternary Science Reviews

Volume 10, Issues 2–3, 1991, Pages 247-258

https://doi.org/10.1016/0277-3791(91)90023-N Get rights and content

Abstract

Detailed seismic stratigraphy, lithostratigraphy, and chemostratigraphy indicate that atoll-wide subaerial exposure surfaces (major disconformities) developed during major sea-level lowstands form prominent seismic reflectors and are coincident with biostratigraphic breaks in the Plio-Pleistocene on Enewetak Atoll. Sea-level models based on the stratigraphic position and age of major disconformities suggest a maximum sea-level highstand elevation of 36 m above present sea level and a maximum sea-level lowstand elevation of 63 m below present sea level for the Pliocene.

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The sediments in zone 5 are now completely composed of LMC, the ∆47 derived temperatures are similar to those in the other sections (Fig. 2A–D), falling close to the temperatures of modern seawater at Enewetak. In addition to the major hiatus associated with prolonged subaerial exposure, zone 5 also contains multiple exposure surfaces (Quinn, 1991b; Wardlaw and Quinn, 1991). Although this zone is present in all of the Enewetak cores, it makes up only a small section of the shorter cores, KAR-1, KBZ-4, and OOR-17.
The geochemical records of shallow-water carbonate sediments can be influenced by both meteoric and marine diagenesis, where the contribution from each can be difficult to constrain using traditional δ¹⁸O and δ¹³C values. Here sediment cores from Enewetak Atoll have been utilized as a case study, in which clumped isotope-derived temperatures and calculated δ¹⁸O_fluid values are combined with the δ³⁴S values of the carbonate-associated sulfate, to differentiate the types of diagenetic alteration which have influenced the geochemical record, thus providing further insights into the extent and timing of carbonate alteration. While sediments at Enewetak Atoll have undergone repeated periods of subaerial exposure, calculated δ¹⁸O_fluid values indicate that the amount of meteoric alteration is reliant on the availability of meteoric fluids, as the presence of exposure surfaces alone does not imply that extensive diagenetic alteration has occurred. Additionally, in the deepest zone of the Enewetak cores, it could be determined that the sediments were initially exposed to meteoric fluids, resulting in partial alteration, and later affected by burial and circulation of deep ocean water, resulting in recrystallization within the section, uniformly overprinting the temperature and δ¹⁸O_fluid values. The results of this study indicate that by utilizing systematics such as δ³⁴S_CAS, ∆₄₇, and δ¹⁸O_fluid values, in addition to the more traditional proxies, a detailed understanding of the diagenetic processes which have altered shallow water sediments can be better constrained, allowing for more accurate interpretations of ancient sedimentary records.
The influence of diagenesis on carbon and oxygen isotope values in shallow water carbonates from the Atlantic and Pacific: Implications for the interpretation of the global carbon cycle
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Global correlation of negative excursions in the δ¹³C values of shallow water carbonates have been used to interpret and identify major environmental changes through geologic time, such as the glaciations of the Neoproterozoic. As a result of their global reproducibility and similar geometry, these signals have predominantly been interpreted as being representative of the original δ¹³C values of dissolved inorganic carbon in the oceans, which in turn reflect major changes in the global carbon cycle. In this paper δ¹³C values from cores drilled in Enewetak Atoll (Pacific) and the Great Bahama Bank (Atlantic), are shown as examples of how similarly correlating variations in global δ¹³C values, can arise from diagenetic processes. It is further shown that while the profiles of the changes in δ¹³C values may appear synchronous, as a result of varying rates of deposition and subsidence, that the actual timing of the two changes can be quite different. By comparison of the geometry and global correlation of δ¹³C values measured in this study, it is proposed that the δ¹³C values of ancient sequences, may have resulted from non-synchronous diagenetic processes, rather than true changes in the global carbon system. This emphasizes the need for precise chronological control when interpreting sediment records, which is not present during many older geological time periods, in order to constrain the origin and correlation of carbon isotope variations.
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The legacy of the Scientific Committee on Antarctic Research’s (SCAR) PAIS strategic research programme includes not only breakthrough scientific discoveries, but it is also the story of a long-standing deep collaboration amongst different multi-disciplinary researchers from many nations, to share scientific infrastructure and data, facilities, and numerical models, in order to address high priority questions regarding the evolution and behaviour of the Antarctic ice sheets (AIS). The PAIS research philosophy is based on data-data and data-model integration and intercomparison, and the development of ‘ice-to-abyss’ data transects and paleo-environmental, extending from the ice sheet interior to the deep sea. PAIS strives to improve understanding of AIS dynamics and to reduce uncertainty in model simulations of future ice loss and global sea level change, by studying warm periods of the geological past that are relevant to future climate scenarios. The multi-disciplinary approach fostered by PAIS represents its greatest strength. Eight years after the start of this programme, PAIS achievements have been high-profile and impactful, both in terms of field campaigns that collected unique data sets and samples, and in terms of scientific advances concerning past AIS dynamics, that have measurably improved understanding of ice sheet sensitivity in response to global warming. Here we provide an overview and synthesis of the new knowledge generated by the PAIS Programme and its implications for anticipating and managing the impacts of global sea-level rise.
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Citation Excerpt :
The first approach is based on observations of the elevation of dated shoreline indicators, with eustatic (globally averaged) sea level assumed to be a direct response to ice mass change. These field-based determinations of paleo-sea level use fundamental principles of geomorphology and morphostratigraphic succession combined with relative and absolute geochronological dating of paleo-shoreline features and intertidal to subtidal facies (Dowsett and Cronin, 1990; Krantz, 1991; Wardlaw and Quinn, 1991; James et al., 2006; Naish and Wilson, 2009; Miller et al., 2005, 2012; Rovere et al., 2014; Dutton et al., 2015; Kemp et al., 2015; Rovere et al., 2016). The early field studies of the MPWP were sometimes corrected for local tectonic effects (typically by reference to well-dated late Pleistocene shorelines) and sometimes for subsidence caused by sedimentary loading (Krantz, 1991).
Understanding the sensitivity of the polar ice caps to a modest global warming (2–3 °C above preindustrial) is of paramount importance if we are to accurately predict future sea level change, knowledge that will inform both social and economic policy in the coming years. However, decades of study of the Pliocene (2.6–5.3 Ma), an epoch in recent Earth history characterized by atmospheric CO₂ levels similar to today, have so far failed to provide definitive sea level and ice volume estimates for that time. Here we review the sources of uncertainty in the paired Mg/Ca-δ¹⁸O methodology used to estimate past sea level, ice volume, and ocean temperature, as well as discuss common assumptions that may bias our interpretation of ocean geochemical records including the LR04 benthic-δ¹⁸O stack, a global compilation of 57 oxygen isotope records that forms the standard template of climate change history for the last 5.3 million years.
Land-use change and managed aquifer recharge effects on the hydrogeochemistry of two contrasting atoll island aquifers, Roi-Namur Island, Republic of the Marshall Islands
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Citation Excerpt :
An unconformity delineates the Holocene and Pleistocene stratigraphic layers, which is termed the “Thurber Discontinuity” and represents a hiatus in carbonate deposition and subsequent diagenesis between approximately 125 ka and 8 ka during low glacio-eustatic sea-levels (Thurber et al., 1965). Studies on Enewetak Atoll (Wardlaw and Quinn, 1991) and Deke Atoll (Ayers and Vacher, 1986) have used drilling cores and seismic interpretation to identify the Thurber Discontinuity. Drilling records from Roi-Namur indicate that consolidated limestone, presumed to be the lower Pleistocene limestone, is located between 7 and 14 m below land surface (bls) (Gingerich, 1992) (Fig. 2).
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The record of Pliocene sea-level change at Enewetak Atoll

Abstract

Cenozoic geochronology

Geological Society of America Bulletin

Drilling operations of Eniwetok Atoll during Project EXPOE

Air Force Weapons Laboratory Technical Report AFWL-TR-75-216

Pacific Enewetak Atoll Crater Exploration (PEACE) Program, Enewetak Atoll, Republic of the Marshall Islands; Part 2: Paleontology and biostratigraphy application to OAK and KOA craters

U.S. Geological Survey Open-File Report 87–665

Insights on why graphic correlation (Shaw's method) works

Journal of Geology

Multichannel seismic-reflection survey of KOA and OAK craters

Disconformities and Sr-isotope stratigraphy reveal a Neogene sea-level history from Enewetak Atoll, Marshall Islands, Central Pacific

Geological Society of America Abstracts with Programs

Ancient sea levels from atoll stratigraphy: the Enewetak model

American Geophysical Union Abstracts with Programs

Reply on “Strontium-isotope stratigraphy of Enewetak Atoll”

Geology

Chronology of fluctuating sea levels since the Triassic

Science

Introduction to geological and geophysical investigations of Enewetak Atoll, Republic of the Marshall Islands

U.S. Geological Survey Professional Paper 1513-A

Pacific Enewetak Atoll Crater Exploration (PEACE) Program, Enewetak Atoll, Republic of the Marshall Islands; Part 1: Drilling operations and descriptions of boreholes in vicinity of KOA and OAK craters

U.S. Geological Survey Open-File Report 86–419

Miocene sea-level falls related to the geologic history of Midway Atoll

Geology