Environmental reconstruction of a Mexican flooded cave system: evidence for climate-forced changes to the local freshwater lens
Abstract
Three radiocarbon-dated sediment cores (4.2 kya to present) from Aktun Ha (Carwash) Cave, Mexico indicate that sediments in flooded caves document Holocene environmental changes. Three distinct environmental phases were determined based on microfossils (shelled rhizopods), organic geochemistry (δ13C, δ15N, TOC, TN, C/N), and particle size distributions (PSDs). In the three cores, terrestrial organic matter was more prevalent in the core closer to the cave entrance (cenote), whereas, aquatic organic matter was more prevalent in cores deeper into the cave. Euryhaline foraminifera (78%) indicate high oligohaline conditions in the freshwater lens (salinity 3.5 - 5.0 psu) during Phase 1 (preceding 4.3 kya), which had higher hydrologic flow rates as interpreted from low organic OM deposition (less than 10%). Phase 2 (4.2 - 2.7 ky BP) is characterized by oscillations between testate amoebae (centropyxids), euryhaline foraminifera (Ammonia tepida var. juvenile), and variable δ15N values. The increases in testate amoebae from Phase 1 and up to 2.5‰ enrichment in δ15N values coupled with poorly sorted PSDs indicate repeated freshening of the freshwater lens (ca. 3.5 - 2.5 psu shift) and associated transport of terrestrial organic matter into the cave. Phase 3 is marked by the colonization of Lagenodifflugia vas at 2.76 ky BP, indicating the onset of modern salinity (1.5 psu) conditions in the local freshwater lens. This is synchronous with a significant reduction in the C/N ratio, and long-term stabilization in sedimentation. The onset of modern hydrologic flow rates in the freshwater lens (less than 1.5 m/s) contributes to increased algae populations in the cenote, increased abundance of organic matter in the cave (TOC greater than 46%). The observed environmental shifts in the cave sediments correlate well with local precipitation changes and storm activity associated with southward migration of the Intertropical Convergence Zone during the late Holocene. Importantly, these results indicate (1) that climate exudes greater control on the phreatic cave environment and aquifer than previously documented, and (2) sediment accumulations in phreatic caves contain viable climate archives, which represent a significantly unexplored source of paleoclimate data.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSMPP74A..04V
- Keywords:
-
- 0458 Limnology (1845;
- 4239;
- 4942);
- 1605 Abrupt/rapid climate change (4901;
- 8408);
- 1641 Sea level change (1222;
- 1225;
- 4556);
- 1803 Anthropogenic effects (4802;
- 4902);
- 4217 Coastal processes