Southwest Pacific subtropics responded to last deglacial warming with changes in shallow water sources

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 29 (2014): 595–611, doi:10.1002/2013PA002584.This study examined sources of mixed layer and shallow subsurface waters in the subtropical Bay of Plenty, New Zealand, across the last deglaciation (~30–5 ka). δ18O and δ13C from planktonic foraminifera Globgerinoides bulloides and Globorotalia inflata in four sediment cores were used to reconstruct surface mixed layer thickness, δ18O of seawater (δ18OSW) and differentiate between high- and low-latitude water provenance. During the last glaciation, depleted planktonic δ18OSW and enriched δ13C (−0.4–0.1‰) indicate surface waters had Southern Ocean sources. A rapid δ13C depletion of ~1‰ in G. bulloides between 20 and 19 ka indicates an early, permanent shift in source to a more distal tropical component, likely with an equatorial Pacific contribution that persisted into the Holocene. At 18 ka, a smaller but similar shift in G. inflata δ13C depletion of ~0.3‰ suggests that deeper subsurface waters had a delayed reaction to changing conditions during the deglaciation. This contrasts with the isotopic records from nearby Hawke Bay, to the east of the North Island of New Zealand, which exhibited several changes in thermocline depth indicating switches between distal subtropical and proximal subantarctic influences during the early deglaciation ending only after the Antarctic Cold Reversal. Our results identify the midlatitude subtropics, such as the area around the North Island of New Zealand, as a key region to decipher high- versus low-latitude influences in Southern Hemisphere shallow water masses.Funding for this project came from NSF OCE-0823487 and 0823549-03.2014-12-1

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

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 (R2 = 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 δ ¹⁸O of seawater (δ ¹⁸Osw) at these sites correlate with instrumental sea surface salinity (SSS; r = 0.64–0.67) but not local precipitation (r = −0.10 to −0.22) demonstrating that the advection and mixing of different salinity water masses may be the predominant control on δ¹⁸Osw 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 δ¹⁸Osw 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

A multi-proxy look at deglacial southwest Pacific Ocean sea surface temperature and thermocline source water changes

Glacial-interglacial changes in global and regional temperature have been linked to changes in insolation, winds, and ocean circulation. This study reconstructs sea surface temperature (SST) and source water provenance across the last deglaciation (~30-5 kyr BP) in five sediment cores in the Bay of Plenty, New Zealand and places these in a regional context by comparing these results to previously published work. SST reconstructions from Mg/Ca ratios in the planktonic foraminifera Globogerina bulloides and alkenones (Ú) track different seasons’ SST spring and summer, respectively. During the last glacial maximum (LGM, 26-22 kyr BP), summer SSTs average 16.4°C while spring SSTs were 13.6; about 4°C cooler than modern. The seasons track well with each other and maintain a constant offset of 3.3-2.8°C as temperatures increase into the Holocene, peaking at 21.7°C for summer and 18.4°C for spring. Comparison to model reconstructions of local insolation yielded correlation to winter insolation from the LGM (~26 kyr BP) to the Antarctic Cold Reversal ACR (~14.1 kyr BP) after which SSTs correlate well to their respective seasonal insolation. Comparison of this study’s temperatures to published SSTs indicate that deglacial warming of subtropical waters differ from subantarctic waters that warmed later and by 2°C more than subtropical waters. δ18O and δ13C from planktonic foraminifera G. bulloides and Globorotalia inflate were used to reconstruct δ18O of seawater (δ18Osw) and track source water provenance. In the LGM, depleted δ18OSW averaging 0.2‰, and enriched δ13C ranging between -0.4-0.1‰ indicate shallow water masses had a strong local Southern Ocean component. A step change occurs at 20.1 kyr BP where δ13C depletes to -1.3‰ that suggests a deglacial shift in shallow subsurface water mass source location to a distal subtropical component likely sourcing through the equatorial Pacific that persists into the Holocene. A regional comparison indicates numerous switches between distal-subtropical and proximal- subantarctic influences during the early deglaciation. This ends at the ACR, which figures as a tipping point for stabilization and onset of modern circulation.M.S.Includes bibliographical referencesby Benedetto Schiraldi Jr

Annual average Sr/Ca measurements from Porites lutea coral at Ha'afera Island, Tonga

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 d18O of seawater (d18Osw) at these sites correlate with instrumental sea surface salinity (SSS; r = 0.64 - 0.67) but not local precipitation (r = -0.10 to - 0.22) demonstrating that the advection and mixing of different salinity water masses may be the predominant control on d18Osw 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 d18Osw 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