Elisabeth L. Sikes

Glacial-interglacial sea surface temperature changes across the subtropical front east of New Zealand based on alkenone unsaturation ratios and foraminiferal assemblages

0 are compared to those derived from foraminiferal assemblages (using the modern analog technique) in two of these cores. Reconstructions of SST in core tops and Holocene sediments agree well with modern average summer temperatures of � 18� C in subtropical waters and � 14� C in subpolar waters, with a 4� -5� C gradient across the front. Down core U37 K 0 SST estimates indicate that the regional summer SST was 4� -5� C cooler during the last glaciation with an SST of � 10� C in subpolar waters and an SST of � 14� C in subtropical waters. Temperature reconstructions from foraminiferal assemblages agree with those derived from alkenones for the Holocene. In subtropical waters, reconstructions also agree with a glacial cooling of 4� to � 14� C. In contrast, reconstructions for subantarctic pre- Holocene waters indicate a cooling of 8� C with glacial age warm season water temperatures of � 6� C. Thus the alkenones suggest the glacial temperature gradient across the front was the same or reduced slightly to 3.5� -4� C, whereas foraminiferal reconstructions suggest it doubled to 8� C. Our results support previous work indicating that the STF remained fixed over the Chatham Rise during the Last Glacial Maximum. However, the differing results from the two techniques require additional explanation. A change in euphotic zone temperature profiles, seasonality of growth, or preferred growth depth must have affected the temperatures recorded by these biologically based proxies. Regardless of the specific reason, a differential response to the environmental changes between the two climate regimes by the organisms on which the estimates are based suggests increased upwelling associated with increased winds and/or a shallowing of the thermocline associated with increased stratification of the surface layer in the last glaciation. INDEX TERMS: 4267 Oceanography: General: Paleoceanography; 4850 Oceanography: Biological and Chemical: Organic marine chemistry; 1050 Geochemistry: Marine geochemistry (4835, 4850); 1055 Geochemistry: Organic geochemistry; KEYWORDS: paleoceanography, sea surface temperature, alkenones, Southern Ocean, Last Glacial Maximum

Southern Ocean seasonal temperature and Subtropical Front movement on the South Tasman Rise in the late Quaternary

[1] The Subtropical Front (STF) marking the northern boundary of the Southern Ocean has a steep gradient in sea surface temperature (SST) of approximately 4C over 0.5 of latitude. Presently, in the region south of Tasmania, the STF lies nominally at 47S in the summer and 45S in the winter. We present here SST reconstructions in a latitudinal transect of cores across the South Tasman Rise, southeast of Australia, during the late Quaternary. SST reconstructions are based on two paleotemperature proxies, alkenones and faunal assemblages, which are used to assess past changes in SST in spring and summer. The north-south alignment in core locations allows reconstruction of movement of the STF over the last 100 ka. Surface water temperatures during the last glaciation in this region were � 4C colder than today. Additional temperature changes greater in magnitude than 4C seen in individual cores can be attributed to changes in the water mass overlying the core site caused by the movement of the front across that location. During the penultimate interglacial, SST was � 2C warmer and the STF was largely positioned south of 47S. Movement of the STF to the north occurred during cool climate periods such as the last marine isotope stages 3 and 4. In the last glaciation, the front was at its farthest north position, becoming pinned against the Tasmanian landmass. It moved south by 4 latitude to 47S in summer during the deglaciation but remained north of 45S in spring throughout the early deglaciation. After 11 ka B.P. inferred invigoration of the East Australia Current appears to have pushed the STF seasonally south of the East Tasman Plateau, until after 6 ka B.P. when it achieved its present configuration.

Upper-ocean-to-atmosphere radiocarbon offsets imply fast deglacial carbon dioxide release

Radiocarbon in the atmosphere is regulated largely by ocean circulation, which controls the sequestration of carbon dioxide (CO2) in the deep sea through atmosphere–ocean carbon exchange. During the last glaciation, lower atmospheric CO2 levels were accompanied by increased atmospheric radiocarbon concentrations that have been attributed to greater storage of CO2 in a poorly ventilated abyssal ocean. The end of the ice age was marked by a rapid increase in atmospheric CO2 concentrations that coincided with reduced 14C/12C ratios (Δ14C) in the atmosphere, suggesting the release of very ‘old’ (14C-depleted) CO2 from the deep ocean to the atmosphere. Here we present radiocarbon records of surface and intermediate-depth waters from two sediment cores in the southwest Pacific and Southern oceans. We find a steady 170 per mil decrease in Δ14C that precedes and roughly equals in magnitude the decrease in the atmospheric radiocarbon signal during the early stages of the glacial–interglacial climatic transition. The atmospheric decrease in the radiocarbon signal coincides with regionally intensified upwelling and marine biological productivity, suggesting that CO2 released by means of deep water upwelling in the Southern Ocean lost most of its original depleted-14C imprint as a result of exchange and isotopic equilibration with the atmosphere. Our data imply that the deglacial 14C depletion previously identified in the eastern tropical North Pacific must have involved contributions from sources other than the previously suggested carbon release by way of a deep Southern Ocean pathway, and may reflect the expanded influence of the 14C-depleted North Pacific carbon reservoir across this interval. Accordingly, shallow water masses advecting north across the South Pacific in the early deglaciation had little or no residual 14C-depleted signals owing to degassing of CO2 and biological uptake in the Southern Ocean.

Southwest Pacific Ocean surface reservoir ages since the last glaciation: Circulation insights from multiple‐core studies

Radiocarbon (C) in dissolved inorganic carbon in the ocean can trace the age of ocean water relative to the atmosphere and provide insight into climate-driven changes in ocean circulation since the last glaciation. Here we estimate surface radiocarbon ages from the last glaciation through the deglaciation into the Holocene in the southwestern Pacific by using tephras, both as stratigraphic tie points and for the availability of existing radiocarbon dates from terrestrialbased analyses of the organic carbon associated with them, as markers of past atmospheric ΔC. The glacial surface reservoir age of subtropical waters was ~700 C years older than the coeval atmosphere at ~25,000 cal yr B.P. This was significantly older (more C depleted) by ~ 300 C years, than modern reservoir ages. At the same time, subantarctic surface water reservoir age was ~3200 C years, almost 5 times the modern reservoir age, making the difference in age between subtropical and subantarctic surface water masses treble the modern difference. This pattern is attributed to the upwelling and exchange of very old deepwaters from the glacial abyss in the Southern Ocean. In the early deglaciation, surface reservoir ages were ~600 to 700 C years. Recent atmospheric ΔC calibrations project that these surface reservoir ages were older than modern by 1.2-fold to 2-fold. This increased reservoir effect can be attributed to shallow circulation that differed from modern, delivering waters with lower C content to the region. Early Holocene surface reservoir ages of ~300 to 500 C years, similar to recent, suggest modern circulation patterns were in place by that time.

Calibration of the carbon isotope composition (δ13C) of benthic foraminifera

The carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6 ± 0.4) × 10−3‰/(μmol kg−1) [CO32−] and pressure (−4.9 ± 1.7) × 10−5‰ m−1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ13CDICnat = δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.

Bacterial influence on alkenones in live microalgae

The microalga Emiliania huxleyi produces alkenone lipids that are important proxies for estimating past sea surface temperatures. Field calibrations of this proxy are robust but highly variable results are obtained in culture. Here, we present results suggesting that algal‐bacterial interactions may be responsible for some of this variability. Co‐cultures of E. huxleyi and the bacterium Phaeobacter inhibens resulted in a 2.5‐fold decrease in algal alkenone‐containing lipid bodies. In addition levels of unsaturated alkenones increase in co‐cultures. These changes result in an increase in the reconstructed growth temperature of up to 2°C relative to axenic algal cultures.

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

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.

Enhanced δ13C and δ18O Differences Between the South Atlantic and South Pacific During the Last Glaciation: The Deep Gateway Hypothesis

Enhanced vertical gradients in benthic foraminiferal δ13C and δ18O in the Atlantic and Pacific during the last glaciation have revealed that ocean overturning circulation was characterized by shoaling of North-Atlantic sourced interior waters; nonetheless our understanding of the specific mechanisms driving these glacial isotope patterns remains incomplete. Here we compare high-resolution depth transects of Cibicidoides spp. δ13C and δ18O from the Southwest Pacific and the Southwest Atlantic to examine relative changes in northern and southern sourced deep waters during the Last Glacial Maximum (LGM) and deglaciation. During the LGM, our transects show that water mass properties and boundaries in the South Atlantic and Pacific were different from one another. The Atlantic between ~1.0 and 2.5 km was more than 1 ‰ enriched in δ13C relative to the Pacific and remained more enriched through the deglaciation. During the LGM, Atlantic δ18O was ~ 0.5 ‰ more enriched than the Pacific, particularly below 2.5 km. This compositional difference between the deep portions of the basins implies independent deep water sources during the glaciation. We attribute these changes to a ‘deep gateway’ effect whereby northern sourced waters shallower than the Drake Passage sill were unable to flow southward into the Southern Ocean because a net meridional geostrophic transport cannot be supported in the absence of a net east-west circumpolar pressure gradient above the sill depth. We surmise that through the LGM and early deglaciation, shoaled northern-sourced waters were unable to escape the Atlantic and contribute to deep water formation in the Southern Ocean.

Deep-sea coral δ13C: A tool to reconstruct the difference between seawater pH and δ11B-derived calcifying fluid pH

The boron isotopic composition (?11B) of coral skeleton is a proxy for seawater pH. However, ?11B-based pH estimates must account for the pH difference between seawater and the coral calcifying fluid, ?pH. We report that skeletal ?11B and ?pH are related to the skeletal carbon isotopic composition (?13C) in four genera of deep-sea corals collected across a natural pH range of 7.89–8.09, with ?pH related to ?13C by ?pH?=?0.029?×??13C?+?0.929, r2?=?0.717. Seawater pH can be reconstructed by determining ?pH from ?13C and subtracting it from the ?11B-derived calcifying fluid pH. The uncertainty for reconstructions is ±0.12 pH units (2 standard deviations) if estimated from regression prediction intervals or between ±0.04 and ±0.06 pH units if estimated from confidence intervals. Our new approach quantifies and corrects for vital effects, offering improved accuracy relative to an existing ?11B versus seawater pH calibration with deep-sea scleractinian corals.

Deep-sea coral δ13C

The boron isotopic composition (?11B) of coral skeleton is a proxy for seawater pH. However, ?11B-based pH estimates must account for the pH difference between seawater and the coral calcifying fluid, ?pH. We report that skeletal ?11B and ?pH are related to the skeletal carbon isotopic composition (?13C) in four genera of deep-sea corals collected across a natural pH range of 7.89–8.09, with ?pH related to ?13C by ?pH?=?0.029?×??13C?+?0.929, r2?=?0.717. Seawater pH can be reconstructed by determining ?pH from ?13C and subtracting it from the ?11B-derived calcifying fluid pH. The uncertainty for reconstructions is ±0.12 pH units (2 standard deviations) if estimated from regression prediction intervals or between ±0.04 and ±0.06 pH units if estimated from confidence intervals. Our new approach quantifies and corrects for vital effects, offering improved accuracy relative to an existing ?11B versus seawater pH calibration with deep-sea scleractinian corals.