Oceanographic variability in the South Pacific Convergence Zone region over the last 210 years from multi‐site coral Sr/Ca records

Abstract

[1] In the South Pacific Convergence Zone (SPCZ), the variability in a sub-seasonally resolved microatoll Porites colony Sr/Ca record from Tonga and a previously published high-resolution record from Fiji are strongly influenced by sea surface temperature (SST) over the calibration period from 1981 to 2004 (R 2 = 0.67-0.68). However, the Sr/Ca-derived SST correlation to instrumental SST decreases back in time. The lower frequency secular trend (~1 C) and decadal-scale (~2-3 C) modes in Sr/Ca-derived SST are almost two times larger than that observed in instrumental SST. The coral Sr/Ca records suggest that local effects on SST generate larger amplitude variability than gridded SST products indicate. Reconstructed d 18O of seawater (d 18 O sw ) at these sites correlate with instrumental sea surface salinity (SSS; r = 0.64-0.67) but not local precipitation (r = À0.10 to À0.22) demonstrating that the advection and mixing of different salinity water masses may be the predominant control on d 18O sw in this region. The Sr/Ca records indicate SST warming over the last 100 years and appears to be related to the expansion of the western Pacific warm pool (WPWP) including an increasing rate of expansion in the last~20 years. The reconstructed d 18 O sw over the last 100 years also shows surface water freshening across the SPCZ. The warming and freshening of the surface ocean in our study area suggests that the SPCZ has been shifting (expanding) southeast, possibly related to the southward shift and intensification of the South Pacific gyre over the last 50 years in response to strengthened westerly winds.

Geochemistry-based coral palaeoclimate studies and the potential of ‘non-traditional’ (non-massive Porites) corals: Recent developments and future progression

“…This difference may act as a significant source of variability in climate reconstructions depending on the diel period over which calcification occurs (Chalker, 1977;Barnes and Crossland, 1978), as lateafternoon peaks in reef flat temperature are unlikely to be representative of the ambient oceanic conditions. This biasing of temperatures in shallow waters has severe implications for attempts at reconstructing absolute temperature based on coral colonies that occur on ponded reef flats, e.g., microatolls (Woodroffe and Gagan, 2000;McGregor et al, 2011a;Wu et al, 2013). To some extent, similar issues may affect data from corals collected from shallow lagoons.…”

“…However, in situ monitoring programs are expensive to set up and maintain (Castillo and Lima, 2010), with limited loggers available in coral reef environments. In light of this, gridded SST datasets acquired by satellites (Castillo and Lima, 2010), or optimally interpolated datasets combining both satellite and in situ observations (Reynolds and Smith, 1994;Reynolds et al, 2002) are commonly used in the calibration processes (e.g., Bagnato et al, 2004;Gallup et al, 2006;Hetzinger et al, 2006;Wu et al, 2013). Satellite derived datasets are cheaper to acquire, and provide a larger spatial representation of SST that can be beneficial to large scale palaeoclimate reconstructions.…”

“…A suite of element/Ca ratios in biological carbonate samples can be analysed rapidly using either inductively coupled plasma-mass spectrometry (ICP-MS) (e.g., Nguyen et al, 2013;Clark et al, 2014), inductively coupled plasma-atomic emission spectrometry (ICP-AES)/inductively coupled plasma-optimal emission spectrometer (ICP-OES) (e.g., Cahyarini et al, 2009;Flannery and Poore, 2013;Wu et al, 2013), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) (e.g., Alibert and Kinsley, 2008;Seo et al, 2013), sector field inductively coupled plasma mass spectrometry (SF-ICP-MS) (e.g., Quinn and Sampson, 2002) or ion microprobe analysis (e.g., Cohen et al, 2004;Meibom et al, 2004). Rapid analytical rates delivered by these techniques provide attractive throughput for palaeoclimate reconstruction as they contrast sharply with the 20-30 minutes required to determine stable isotope ratios in a single sample using an isotope ratio mass spectrometer (IR-MS) (Quinn and Sampson, 2002).…”

Intra-skeletal calcite in a live-collected Porites sp.: Impact on environmental proxies and potential formation process

“…In tropical settings, acquisition of paleo-SST are currently obtained using δ 18 O and/or trace elements from aragoniticskeletonof scleractinian corals, mostly Porites sp. (e.g., Alibert and McCulloch, 1997;Beck et al, 1992;Corrège, 2006;Liu et al, 2009;Wu et al, 2013). In the massive coral Porites sp., the Sr/Ca ratio is the most commonlyused proxy for SST reconstructions (e.g., Corrège, 2006;Gagan et al, 1998;Zinke et al, 2014).…”

“…Contrarily, boron isotopes in Porites sp. skeleton has been used with success to reconstruct paleo-pH of surface seawater and ocean acidification related to the inception and development of the industrial era (e.g., Douville et al, 2010;Liu et al, 2009;Shinjo et al, 2013a;Wei et al, 2009;Wu et al, 2013). Even if boron isotopes in corals are now considered to be directly the reflect of pH of the calcification fluid (Allison et al, 2010;Blamart et al, 2007;Rollion-Bard et al, 2003;Venn et al, 2011), Trotter et al(2011) highlighted that pH of the calcification in scleractinian corals depends on pH of seawater.…”