
Earth Sciences
Higher than present global mean sea level recorded by an Early Pliocene intertidal unit in Patagonia (Argentina)
A. Rovere, M. Pappalardo, et al.
Explore groundbreaking research led by Alessio Rovere and colleagues that reveals astonishing insights into global mean sea level during the Early Pliocene. Their findings suggest that sea levels may have been over 20 meters higher than today, hinting at a dramatically different Earth without significant ice cover. This study from central Patagonia uncovers crucial historical data for understanding future sea level rise.
Playback language: English
Introduction
Understanding past sea levels during warmer periods is crucial for predicting future sea level rise. Paleo relative sea level (RSL) indicators, such as fossil reefs or beach deposits, provide direct proxies for estimating past GMSL. However, these indicators require correction for factors like tectonics, mantle dynamic topography (DT), and glacial isostatic adjustment (GIA) to obtain accurate GMSL estimates. Well-preserved and accurately dated RSL indicators are scarce for periods older than 400,000 years ago due to poor preservation and dating challenges. This limits our ability to assess ice sheet sensitivity to warmer climates, particularly those of the Pliocene epoch. Recent studies have reported precise Early Pliocene RSL indicators in Mallorca (Spain) and South Africa, suggesting high sea levels at that time. These studies, however, are few, making further investigation into other regions essential to strengthen our understanding of GMSL during the Early Pliocene. The Early Pliocene is particularly relevant as a potential analog for future warmer climates, given that CO2 levels were intermediate between pre-industrial and modern levels, and global temperatures were 2-3°C higher than pre-industrial values. Ice models suggest that during the warmest Pliocene interglacials, Greenland was ice-free, and the West and potentially East Antarctic Ice Sheets were significantly smaller than today. This study focuses on a new Early Pliocene RSL indicator from Patagonia, Argentina to contribute to this limited dataset and refine our understanding of GMSL during the Early Pliocene warm period.
Literature Review
Previous studies have attempted to reconstruct global mean sea level during the Early Pliocene using various methods, including oxygen isotope records and direct measurements of paleo sea level indicators. Indirect methods, such as those using benthic oxygen isotopes, provide useful long-term trends, but lack the precision of direct measurements. Recent work on Mallorca, Spain and South Africa has provided some of the first precise and well-dated direct observations of sea levels during the Early Pliocene. The Mallorca study analyzed phreatic overgrowths on speleothems in a coastal cave, dating to 4.29 ± 0.39 Ma, indicating a GMSL approximately 25.1 m above present. In South Africa, oyster shell deposits dating from 4.28-4.87 Ma provided evidence for a paleo RSL, suggesting that with minor vertical land movements, the area may have experienced high sea level during the Early Pliocene. These studies offer crucial insights, but their limited geographical representation necessitates further investigation to obtain a robust global perspective. Several ice sheet models exist providing estimates of past sea level changes, but these models often differ in their predictions and are challenged by the limited and uncertain data available for validation.
Methodology
This study investigates a foreshore (intertidal) sequence located in Camarones, central Patagonia, Argentina. The methodology involves a multi-faceted approach combining field data acquisition, strontium isotope stratigraphy (SIS) dating, and the application of Glacial Isostatic Adjustment (GIA) and Dynamic Topography (DT) models to correct for departures from eustasy.
**Elevation Measurements and Paleo RSL Estimates:** Elevations were measured using a differential GPS system, providing accurate ellipsoid and orthometric heights (heights above the GEOIDEAR16 geoid, the best approximation for present sea level in Argentina). The average elevation of the intertidal unit (Unit Cp) was determined at 36.2 ± 0.9 m above the geoid. Considering the modern tidal range of 5 m, paleo RSL was estimated at 36.2 ± 2.7 m above present, assuming no post-depositional movement.
**Age Determination:** Three oyster shells from the intertidal unit were analyzed using strontium isotope stratigraphy (SIS). This involved sequential leaching to target the least altered carbonate, resulting in multiple SIS ages. The analysis yielded an age range of 4.69–5.23 Ma (2σ). Strict screening techniques were applied to ensure the reliability of the selected samples.
**Glacial Isostatic Adjustment (GIA):** GIA corrections were calculated using 36 different Earth models, accounting for both the ongoing response to the last deglaciation and Antarctic ice sheet oscillations during the Early Pliocene. The GIA correction for Camarones was calculated at -14.6 ± 3.2 m.
**Vertical Land Motions (VLMs):** To account for VLMs caused by tectonics, mantle dynamic topography, or sediment loading, predictions from two DT models (Flament et al. and Müller et al.) were considered. A slight long-term uplift trend was observed, resulting in a downward correction of the GMSL inference by 22.4 ± 11.0 m. The high uncertainty in the DT models highlights a limitation of the study.
**Global Mean Sea Level (GMSL) Calculation:** Combining the RSL, GIA, and VLM corrections, the study calculated a GMSL estimate of 28.4 ± 11.7 m above present during the deposition of the intertidal unit. The uncertainties were propagated through the calculations. This value, however, is highly uncertain, primarily due to the limitations in VLMs estimates.
The methodology employed stringent quality control measures, including detailed stratigraphic descriptions and sample screening for SIS analysis, to ensure robust and reliable results.
Key Findings
The key findings of the study include:
1. **Early Pliocene Age:** The intertidal unit at Camarones, Patagonia, was accurately dated to the Early Pliocene (4.69–5.23 Ma) using strontium isotope stratigraphy (SIS). The high-resolution dating method ensured precise chronological placement of the geological formation.
2. **High Paleo RSL:** The unit's elevation of approximately 36 m above present-day sea level indicates a high paleo relative sea level (RSL). The detailed elevation measurements via differential GPS, along with consideration of tidal ranges, provided a robust RSL estimate.
3. **High Early Pliocene GMSL:** After correction for GIA and VLMs, the study estimates that Early Pliocene GMSL was 28.4 ± 11.7 m (1σ) higher than present. While this estimate is uncertain, particularly due to the large range of uncertainty in vertical land movement estimates, the value significantly exceeds present-day sea level.
4. **Consistency with Other Sites:** The Camarones GMSL estimate aligns with those derived from analogous Early Pliocene sea level proxies in the Mediterranean Sea (Mallorca) and South Africa, suggesting that globally, high sea levels during the early Pliocene were widely distributed, lending support to the overall reliability of the findings.
5. **Ice Sheet Implications:** The high GMSL values estimated suggest a scenario involving an ice-free Greenland, extensive melting of the West Antarctic Ice Sheet (WAIS), and significant melting of marine-based sectors of the East Antarctic Ice Sheet (EAIS). This supports prior hypotheses that Antarctic ice sheet contributions to GMSL were substantial during Early Pliocene interglacial periods. The high GMSL value implies significant ice sheet melting globally.
Discussion
The results of this study provide strong evidence for exceptionally high GMSL during the Early Pliocene, significantly exceeding present-day levels. The consistency between the GMSL estimates from Camarones, Mallorca, and South Africa, despite their geographical separation and differences in methodology, strongly supports the reliability of these findings. The high GMSL values indicate substantial melting of major ice sheets. While uncertainty remains, particularly regarding the correction for vertical land motion, the findings strongly suggest an ice-free Greenland and considerable contributions from Antarctic ice sheets. The comparison with other GMSL estimates from ice sheet models and indirect proxies highlights the importance of field-based evidence in constraining past sea levels. This study reinforces the potential for significant future sea level rise under continued warming, particularly if anthropogenic emissions are not curtailed, emphasizing the need for robust climate mitigation efforts. The large error bars associated with the VLM estimations highlight the need for improvements in dynamic topography modeling and further research to refine estimates of past vertical land movement. The utilization of independent GIA models and DT models, with careful propagation of uncertainties, strengthens the overall reliability of this research.
Conclusion
This study presents compelling evidence of significantly higher global mean sea levels during the Early Pliocene, exceeding 20 meters above present-day levels. This highstand is supported by independent paleo sea-level indicators from three continents and points to extensive melting of the Greenland and Antarctic ice sheets. While uncertainties remain in the estimation of vertical land motions, the study emphasizes the importance of field-based evidence in constraining past sea levels and underscores the potential for substantial future sea-level rise under continued warming. Future research should focus on improving models of dynamic topography and expanding the geographical coverage of precise Early Pliocene sea-level records.
Limitations
The main limitation of this study lies in the uncertainties associated with vertical land motion (VLM) corrections. The variability in published dynamic topography models leads to considerable uncertainty in the GMSL estimations. Improved mapping of the mantle structure beneath Patagonia and a better understanding of mantle rheology are needed to reduce this uncertainty. The assumption of linear extrapolation of uplift rates over millions of years is another limitation. Additionally, the study assumes that tidal ranges were similar to modern ones in the Early Pliocene, which may not be entirely accurate. While the meticulous methodology employed provides relatively precise RSL data, the uncertainties related to VLM corrections necessitate careful interpretation of the results and further research to refine the GMSL estimates.
Related Publications
Explore these studies to deepen your understanding of the subject.