Changes in the position of the Subtropical Front south of New Zealand since the last glacial period

Abstract

This study fills an important gap in our understanding of past changes in the Southern Subtropical Front (S-STF) in the southwest Pacific Ocean. Paleo-sea surface temperatures (SST) were estimated from planktic foraminiferal census counts from cores straddling the modern S-STF in the Solander Trough, south of New Zealand. The estimated SST were compared for 6 time slices; glacial period (25-21 ka), Last Glacial Maximum (LGM; 21-18 ka), early deglaciation (18-16 ka), late deglacial/early Holocene period (14-8 ka), mid-Holocene period (8-4 ka), and late Holocene period (4-0 ka). The position of the S-STF was determined by two methods:(1) the location of the 10°C isotherm and (2) the location of the highest SST gradients. These new results suggest that the S-STF was not continuous between east and west of New Zealand during the glacial period. Steep SST gradients indicate that a strong S-STF rapidly shifted south during the LGM and early deglaciation. During the late deglacial and Holocene periods the position of the S-STF differs between the two methods with reduced SST gradients, suggesting a more diffuse S-STF in the Solander Trough at this time. The glacial SST data suggest that the S-STF shifted north to the west of New Zealand, while to the east there was a stronger SST gradient across the front. This was possibly the result of an increased wind stress curl, which could have been caused by stronger, or more northerly Southern Hemisphere westerly winds (SHWW), or a merging of the SHWW split jet in this region.

Modern, Preindustrial, and Past (Last 25 ka) Carbon Isotopic (δ13C) Variability in the Surface Waters of the Southwest Pacific

“…Higher δ 13 C G . bulloides TC values are found in low nutrient STSW of the southwest Pacific from 34–43°S, and two peaks are evident at ~45°S and ~50°S, likely reflecting the position of the highly productive STF to the south and east of New Zealand where the core tops are clustered (Figure ; Bostock et al, ), thus crossing the STF twice. The latitudinal differences in the mean δ 13 C G .…”

“…bulloides TC during the glacial. There is evidence from SST proxies that the STF did not sit south of the South Island during the last glacial (Bostock et al, ) but remained topographically constrained to the Chatham Rise (Sikes et al, ; Weaver et al, ). Therefore, the SCR, SolN, and SolS regions were all overlain by glacial SASW.…”

“…The majority of the stable isotopes were run on G . bulloides tests from the 250‐ to 355‐μm size fraction for the TAN cores, JP cores, and the MD97‐2120 (Bostock et al, ; Pahnke et al, ; Schiraldi et al, ); however, different size ranges were used for a couple of the cores: H214 used 180–300 μm (Samson et al, ); ODP 1123 and CHAT10K cores were run using 300–355 μm and for MD97‐2121 the >150‐μm size range (Carter et al, ). The core tops and cores were grouped into five different regions (Figure a and Tables and ).…”

Late Holocene intensification of the westerly winds at the subantarctic Auckland Islands (51° S), New Zealand

“…7 and 8), which would correspond to fjord bottom water temperature changes greater than 4 • C (∼ +0.23 ‰ per 1 • C increase; Marchitto et al, 2014). This is unlikely given regional terrestrial temperature reconstructions (McGlone et al, 2010), and the < 0.5 ‰ range of Holocene δ 18 O variability in planktic foraminifera derived from a transect of sediment cores collected in the Solander Trough close to the Auckland Islands (Bostock et al, 2015). Therefore, a combination of local salinity effects, temperature changes, and the δ 18 O of regional waters likely influences the δ 18 O of benthic foraminifera in Hanfield Inlet.…”

“…Although the STF southwest of New Zealand shifted rapidly south towards the subantarctic during the deglaciation in response to an intensification/southward shift in SHWW, the position of this front has remained relatively stable over the Holocene (Bostock et al, 2015), likely due to topographic steering at the 500 m depth contour . The efficiency of the SAZ carbon sink is sensitive not only to latitudinal migrations of winds and associated fronts on glacial-interglacial timescales but also to decadal changes in atmospheric circulation patterns, particularly the symmetry of low-level zonal winds (Le Quéré et al, 2007;Metzl, 2009;Takahashi et al, 2009;Landschützer et al, 2015).…”