Millennial-scale reconstruction of Orinoco run-off strength and its impact on the Atlantic Meridional Overturning Circulation (MORA)
Final Report Abstract
Within the framework of this project the factors driving northern South American hydrological variations during the past 25 kyrs have been investigated. Key goal has been to disentangle the interplay of abrupt forcing exerted by changes in the global ocean circulation from gradual forcing imposed by changes in insolation. To reconstruct continental moisture variability, Orinoco river run-off has been reconstructed by determining Ba/Ca ratios on planktonic foraminifera in combination with sedimentological proxies such as sediment reflectance. Foraminiferal Ba/Ca is a potentially powerful tracer for river run-off as the Ba/Ca signature of river water is higher than that of sea water by an order of magnitude. The applicability of this relatively novel proxy has been ground-truthed in an initial study analyzing the test geochemistry of six planktonic foraminifera species derived from core-tops and sediment traps in the vicinity of the Orinoco Delta. The results showed varying sensitivity of the symbiont-bearing species to freshwater influence, depending on the freshwater tolerance and habitat depth of the respective species. Highest sensitivity to river-water input was shown by Globigerinoides ruber (pink and sensu latu varieties), followed by Globigerinoides sacculifer and Orbulina universa. Notably, Ba/Ca ratios do not covary with calculated δ18Oseawater on the same samples, a parameter commonly used to reconstruct sea surface salinities. The lack of a clear river-water signal in δ18Oseawater is due to the similarity of the marine and freshwater δ18Owater endmembers in tropical settings. Instead, δ18Oseawater reflects the on-site precipitation-evaporation balance. Exceptionally high Ba/Ca ratios were found for the non-symbiont bearing, heterotrophic deep dwellers Neogloboquadrina dutertrei and Globorotalia menardii which likely reflect their preference for Acantharians that incorporate high amounts of Barium in their celestine skeleton. Based upon the initial proxy-evaluation study, run-off reconstructions have been performed for the past 25 kyrs using Ba/Ca rations of tests from G. ruber (pink) on sediment core M78/1-235-1 from off the Orinoco Delta. The results indicate an unexpected non-linear response of hydrological variability within the Orinoco catchment to gradual insolation forcing. This is particularly evident during the early Holocene, where Ba/Ca ratios increase abruptly when northern hemisphere summer insolation reached its maximum, and show a likewise sharp decline when insolation decreases. This non-linear behavior of Ba/Ca can be attributed to the activation of a moisture source in the northernmost Andes at the time when the Intertropical Convergence Zone (ITCZ) reached its northernmost position during maximum summer insolation. Here, orographic blocking enhanced rainfall in the northern Andes, while the Ba/Ca composition of the Orinoco was further enhanced by excess Ba2+ derived from rivers draining freshly exposed rocks in the northern Andes. Deglacial Ba/Ca is elevated at core M78/1-235-1 during Heinrich Stadial 1, however, high sediment reflectance argues against the Orinoco as the major source for Ba2+. In accord with published data it appears more likely that the high Ba/Ca ratios reflect a remote freshwater source derived from the Amazon or smaller rivers in East Brazil. A southern freshwater source is in line with a maximum southward position of the ITCZ during states of weak Atlantic Meridional Overturning Circulation such as HS1. Interestingly, the Ba/Ca peak recorded off the Orinoco coincides with elevated Ba/Ca values in Florida Straits, indicating a possible advection of the freshwater plume into the source region of the Gulf Stream.
Publications
- (2013), Comparison of Ba/Ca and δ18OWATER as freshwater proxies: A multi-species core-top study on planktonic foraminifera from the vicinity of the Orinoco River mouth, Earth and Planetary Science Letters, 383, 45-57
Bahr, A., Schönfeld, J., Hoffmann, J., Voigt, S., Aurahs, R., Kucera, M., Flögel, S., Jentzen, A., and Gerdes, A.
(See online at https://doi.org/10.1016/j.epsl.2013.09.036) - (2013), Foraminiferal Ba/Ca geochemistry as tracer for Early Holocene Orinoco discharge, International Conference on Paleoceanography, Sitges, Spain
Hoffmann et al.
- (2014), Abrupt Late Pleistocene changes in northern South American river runoff, AGU Fall Meeting, San Francisco, USA
Hoffmann et al.
- (2014), Disentangling abrupt deglacial hydrological changes in northern South America: Insolation versus oceanic forcing: Geology, 42(7), 579-582
Hoffmann, J., Bahr, A., Voigt, S., Schönfeld, J., Nürnberg, D., and Rethemeyer, J.
(See online at https://doi.org/10.1130/G35562.1) - (2014), Late Pleistocene changes in northern South American river run-off, GeoFrankfurt, Frankfurt am Main
Hoffmann et al.
- (2014), The geochemical analysis of carbonates as a tool for reconstructing past environmental dynamics: studies on seep carbonates and foraminiferal calcite, pp. 259 (Habilitation-Thesis, University of Frankfurt)
Bahr, A.
- (2015), Millennial-scale reconstruction of Orinoco and Amazon River run-off strength and its impact on Atlantic Meridional Overturning Circulation (AMOC) based on foraminiferal Ba/Ca ratios, pp. 120 (PhD-Thesis, University of Frankfurt)
Hoffmann, J.
