Cocos (Keeling) Corals Reveal 200 Years of Multidecadal Modulation of Southeast Indian Ocean Hydrology by Indonesian Throughflow

Abstract

The only low latitude pathway of heat and salt from the Pacific Ocean to the Indian Ocean, known as Indonesian Throughflow (ITF), has been suggested to modulate Global Mean Surface Temperature (GMST) warming through redistribution of surface Pacific Ocean heat. ITF observations are only available since ~1990s, and thus, its multidecadal variability on longer time scales has remained elusive. Here we present a 200 year bimonthly record of geochemical parameters (δ18O‐Sr/Ca) measured on Cocos (Keeling) corals tracking sea surface temperature (SST; Sr/Ca) and sea surface salinity (SSS; seawater‐δ18O═δ18Osw) in the southeastern tropical Indian Ocean (SETIO). Our results show that SETIO SSS and δ18Osw were impacted by ITF transport over the past 60 years, and therefore, reconstructions of Cocos δ18Osw hold information on past ITF variability on longer time spans. Over the past 200 years ITF leakage into SETIO is dominated by the interannual climate modes of the Pacific Ocean (El Niño—Southern Oscillation) and Indian Ocean (Indian Ocean Dipole). Pacific decadal climate variability (represented by the Pacific Decadal Oscillation) significantly impacted ITF strength over the past 200 years determining the spatiotemporal SST and SSS advection into the Indian Ocean on multidecadal time scales. A comparison of our SETIO δ18Osw record to GMST shows that ITF transport varied in synchrony with global warming rate, being predominantly high/low during GMST warming slowdown/acceleration, respectively. This hints toward an important role for the ITF in global warming rate modulation.

Calibration of Sr/Ca, Li/Mg and Sr‐U Paleothermometry in Branching and Foliose Corals

“…To date, Sr/Ca has been the most widely used coral paleothermometer (Figure 6b and supporting information Table S1; Bagnato et al, 2004;Calvo et al, 2007;Corrège et al, 2000;Fallon et al, 1999Fallon et al, , 2003Flannery et al, 2018;Gagan et al, 1998;Heiss et al, 1997;Hennekam et al, 2018;Jimenez et al, 2018;Kawakubo et al, 2017;Linsley et al, 2000;Marshall & McCulloch, 2001;McCulloch, 1999;Mitsuguchi et al, 1996;Nurhati et al, 2009;Ramos et al, 2017;von Reumont et al, 2018;Shen et al, 1996;Sinclair et al, 1998;Watanabe et al, 2017;Wu et al, 2013Wu et al, , 2014Zinke et al, 2004). However, separating the effects of temperature from the influence of vital effects on Sr/Ca has long proven difficult (Alibert & McCulloch, 1997;Corrège, 2006;de Villiers et al, 1995).…”

20th Century δ18O Seawater and Salinity Variations Reconstructed From Paired δ18O and Sr/Ca Measurements of a La Reunion Coral

“…For example, the ITF was shown to respond to the 1975 phase change of the PDO with a reduction in water mass transport (Wainwright et al, ), and coral luminescence records from the SW Indian Ocean show a PDO signature in regional rainfall (Grove, Zinke, et al, ). Moreover, Hennekam et al () showed that variations of the PDO appear to influence Indian Ocean δ 18 O sw and SSS via the ITF, based on a 200‐year coral record from Cocos (Keeling). Cocos (Keeling) is located in the eastern Indian Ocean, within the path of the SEC and close to the ITF exit passages.…”

“…Coral skeletons contain a suite of isotopic and trace elemental indicators that provide high resolution (seasonal to monthly) proxy records spanning several decades to centuries and are an ideal archive to reconstruct past changes in SSS (e.g., Cahyarini et al, 2008;Cahyarini et al, 2014;Felis et al, 2018;Hennekam et al, 2018;Nurhati et al, 2011). The coral oxygen isotopic composition (δ 18 O) is sensitive to SST and the δ 18 O of seawater (δ 18 O sw ), while coral Sr/Ca ratios primarily reflect changes in SST (e.g., Corrège, 2006).…”

“…In the tropical oceans, δ 18 O sw is linearly related to SSS (Schmidt, 1998;Srivastava et al, 2007), as both increase with evaporation and decrease through admixture of low-δ 18 O freshwater. δ 18 O sw estimated from coral skeletons is therefore used to infer decadal to centennial changes of SSS (Cahyarini et al, 2014;Felis et al, 2018;Hennekam et al, 2018;Nurhati et al, 2011). Given the lack of historical SSS data, the reconstruction of δ 18 O sw is an important contribution of coral skeletal archives for our understanding of ocean circulation and climate variability.…”

“…Given the lack of historical SSS data, the reconstruction of δ 18 O sw is an important contribution of coral skeletal archives for our understanding of ocean circulation and climate variability. However, at present only a handful of coral δ 18 O sw and SSS reconstructions are available from the tropical Indian Ocean (Cahyarini et al, 2014;Hennekam et al, 2018;Watanabe et al, 2017;Zinke et al, 2008). While interannual changes of coral δ 18 O sw have been linked to changes in the surface freshwater balance driven by interannual modes of climate variability such as ENSO and the Indian Ocean Dipole (e.g., Abram et al, 2008;Cahyarini et al, 2014;Watanabe et al, 2019), the drivers of lowerfrequency changes of δ 18 O sw are not always known.…”