Coral-based reconstruction of climatic and oceanographic variability across the Luzon Strait for the past ~230 years

• Main Author: Ramos Riovie Dela Pena
• Other Authors: Nathalie Goodkin
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Science::Geology
• Online Access: https://hdl.handle.net/10356/106562; http://hdl.handle.net/10220/47946
• Institution: Nanyang Technological University

The Kuroshio-dominated water exchange across the Luzon Strait impacts the climate and oceanography of the South China Sea and the total transport of the Indonesian Throughflow, inviting interest for research that will improve our understanding of the behavior and drivers of the Kuroshio Intrusion. However, continuous high-resolution observations of the Kuroshio intruding process are limited as comprehensive and systematic monitoring remain logistically challenging and complex. Massive corals provide an inexpensive means of extending high-resolution climatic and oceanographic records of the Kuroshio Intrusion and its drivers beyond the instrumental period, which is critical for evaluating long-term variability, regional trends and predictability under future change. One Diploastrea heliopora and two Porites sp. coral cores were collected on either side of the Luzon Strait – Palaui, northwestern Philippines and Houbihu, southern Taiwan. The coral cores were slabbed and x-rayed to determine sub-annual bands allowing for precise age-control. The slabs were subsampled for Sr/Ca, d18O and Δ14C as proxies for sea surface temperature (SST), sea surface salinity (SSS) and ocean circulation, respectively. Monthly-resolved SST and SSS reconstructions spanning the last 132 and 230 years were generated from Palaui and Houbihu, respectively. Seasonal and annual Δ14C records from both Palaui and Houbihu cover the past ~68 years. Sr/Ca-based records of SST reveal site-specific controls on observed SST variability. The Palaui paired Diploastrea and Porites winter Sr/Ca-SST records exhibit similar sensitivity to the Pacific Decadal Oscillation (PDO). The extended Porites-based winter SST reconstruction since the late 19th century remained sensitive to the PDO, highlighting its potential long-term impact on tropical climate variability. Houbihu winter SST, on the other hand, exhibits hemispheric-scale SST patterns driven by global forcings (i.e. solar and volcanic radiative forcings). The additional ~100 years in Houbihu relative to the Palaui record allow for further examination of conditions at the end of the Little Ice Age (LIA), which indicates cooler SSTs relative to the most recent 30 years. Both coral sites exhibit significant recent rise in SSTs likely tied to anthropogenic warming. Collectively, these findings suggest a change in temperature and extent of the warm pool region in the past 230 years. d18Oc-based records of SSS are robust allowing for investigating the advection signal across the Luzon Strait. Similar to Sr/Ca-SST, each of the Luzon Strait sites reveal site-specific controls on the observed surface salinity variability. The Palaui dry-season SSS record is sensitive to ENSO-related changes in hydrological processes. Seasonal difference between Palaui dry and wet season SSS record reveals the additional impact of the PDO in enhancing advection of Pacific waters to Palaui. In contrast, the Houbihu dry-season SSS record reflects East Asian Winter Monsoon (EAWM)-related changes in hydrological processes, modifying the surface salinity of the source waters. Examining the dry-season SSS difference between Palaui and Houbihu reveals the added influence of the PDO on the delivery of Pacific waters across the strait at decadal timescales. The additional ~100 years of dry-season SSS record in Houbihu indicate high salinity surface water conditions (i.e. drier) at the end of the LIA relative to the present. Inferring from the strong modern relationships between the Houbihu SSS and the EAWM and the PDO, the Kuroshio intrusion at the end of the LIA is likely reduced. These conditions potentially resulted in an altered SCS climate and circulation. Records of Δ14C from both Palaui and Houbihu also reveal site-specific processes that control advection and water mixing at these sites, corroborating findings in the Sr/Ca and d18O-based climate records. Palaui Δ14C relative to Pacific Δ14C values is sensitive to the ENSO-forced migration of the North Equatorial Current (NEC) bifurcation latitude. Houbihu Δ14C relative to SCS Δ14C values reflects EAWM-mediated transport across the Luzon Strait and circulation of the SCS. Overall, the interannual Δ14C difference between our sites and the surrounding seas reveal the dominant influence of ENSO in driving the Kuroshio Intrusion into the SCS at interannual timescales. Across proxy records, this thesis has revealed site-specific drivers of observed climatic variability between sites in the Luzon Strait. This key finding underscores the importance of investigating and identifying processes that are related to EAWM, ENSO and PDO variability independently at each site. Collectively, this information is critical to understand their interaction that in turn controls the Kuroshio intruding process into the SCS.