Response of the Southern Ocean to millennial-scale climate change during marine isotope stage three
Creators: name: Anderson, HJ
Publisher: name: University of Tasmania
Year: 2024

description: Ice core records show that Antarctica rapidly warmed and cooled on centennial to millennial-timescales during marine isotope stage 3 (MIS-3). These warming events are called Antarctic Isotope Maxima (AIM) and are linked to opposing counterparts in the Northern hemisphere — called Dansgaard-Oeschger (D-O) events — through slowdowns of the Atlantic meridional overturning circulation (AMOC). Southern Hemisphere warming during AIM warming events is associated with increasing atmospheric CO2, which the Southern Ocean is thought to play a significant role in modulating. This thesis explores the role of Southern Ocean processes that affect the carbon cycle and climate change during MIS-3 by compiling high-resolution MIS-3 sea surface temperature records, developing a new record of biological and physical changes in the Subantarctic southwest Pacific sector of the Southern Ocean, and using these data to validate a coupled ocean-atmosphere and general circulation modelling.In Chapter two of this thesis, a compilation of 14 high-resolution marine sediment core sea surface temperature (SST) records finds a zonally consistent warming of the Southern Ocean during MIS-3 AIM warming events. The age models of these SST records were harmonised to facilitate comparison by first, recalibrating radiocarbon dates to the MARINE20 calibration, and the application of new tie-points to the EPICA Dome Concordia dD record on the Antarctic Ice Core Chronology 2012 (AICC2012). A 500-yr binned SST stack of all the records shows consistent temperature increases during AIM warming events that are positively correlated with the amplitude of warming at EPICA Dome C (n=10, r2 = 0.65, p = &lt; 0.01). Additionally, the amplitude of Southern Ocean warming scales linearly with the length of the corresponding stadial period in Greenland, indicating that there is no difference in the rate of warming between Heinrich and Non-Heinrich associated AIM events. The consistent warming across the Southern Ocean supports the thermal seesaw hypothesis — a slow build-up of heat in the South Atlantic and subantarctic Southern Ocean during AMOC slowdowns — solidifying the linkage between Southern Ocean sea surface temperatures and Antarctic temperature variability.In the subantarctic Atlantic Ocean, biological and physical processes during MIS-3 AIM warming events have been linked to changes in atmospheric CO2. However, little attention has been paid to other sectors of the Southern Ocean. Chapter three reconstructs millennial scale biological and physical changes in the southwest Pacific sector of the Southern Ocean, using sediment core TAN1106-28 (2798m; 48.37°S, 165.66°E). To achieve this, three sea surface temperature proxies were combined (foraminifera modern analogue transfer function, alkenone Uk’37, and planktic foraminifera Mg/Ca ratios) and compared with, thorium-230 normalised lithogenic material (estimated from thorium-232) and iron flux, export production proxies (thorium-230 normalised fluxes of excess barium, chlorins, and total organic carbon), planktic foraminifera bound d15N, benthic–planktic 14C age offsets, and deep ocean redox proxies (authigenic uranium and excess manganese). TAN1106-28 SST covaries with Antarctic temperature and CO2 during MIS-3. The d15N also covaries with temperature, which is interpreted to reflect nitrate supply changes linked to southward migrating and intensifying Southern Hemisphere westerly winds. Iron flux and export production are strongly correlated and support iron limitation and reduced drawdown of CO2 during late MIS-3 (48–29 ka) AIM warming events. Reduced export production and iron flux during early MIS-3 (60–48 ka) coincides with high SST and glacial retreat in Southern New Zealand, highlighting that glacial erosion may drive bioavailable iron supply and export production. Decreases in bottom water oxygen (BWO) and increases in the benthic–planktic 14C age offset are indicative of an older water mass influencing the southwest Pacific during AIM warming events. I interpret these results as an expansion of Pacific Deep Water into the Southern Ocean during AIM warming events in response to strengthening Southern Ocean convection and increased wind-driven upwelling. These findings are consistent with a less efficient biological pump during early MIS- 3, and during late MIS-3 AIM warming events, and the ventilation of deep-stored carbon to the atmosphere from the Southern Ocean during all MIS-3 AIM warming events.In the final data chapter, Chapter four, I compare the proxy data from chapters 2 and 3 using output from a community earth system general circulation model (CESM-1) glacial experiment (GLA) to understand the possible processes and drivers of these millennial-scale changes. The model is set up using last glacial maximum boundary conditions and spontaneously simulates two D-O-like climate events. GLA SST warming rates are consistent with those in the Atlantic and zonally average SO SST stacks. However, Pacific GLA warming rates exceed sediment core reconstructions, and in Antarctica GLA warming rates are 1.1°C/kyr lower than at EPICA Dome C. The reduced Antarctic warming rate may represent the incomplete transport of heat across the Antarctic Circumpolar Current as a result of SO convection that peaks early during GLA AIM warming events. Additionally, reductions in aeolian iron flux are necessary (using the IRON offshoot from the GLA experiment where iron is reduced by 50%) to better replicate sediment core reconstructed decreases in export production. However, export production decreases remain smaller than paleo-reconstructions, which may be due to the lack of a transient decrease in iron bioavailability during AIM warming events. The GLA experiment shows decreasing BWO in Pacific Circumpolar Deep Water (CDW; 2500–3000 m), in agreement with TAN1106-28 results. However, modelled decreases in Atlantic CDW BWO (3500–4000 m) do not agree with increased BWO in Atlantic sediment cores during AIM warming events. BWO variability in GLA is however, very strongly correlated to GLA Southern Ocean overturning strength (r2 = 0.87, p = &lt;0.01), which peaks at the onset of AIM warming. During peak Southern Ocean overturning, GLA BWO variability supports an expansion of Pacific Deep Water into the Pacific SO via Upper Circumpolar Deep Water, and increased ventilation of Lower Circumpolar Deep Water in the South Atlantic. This pattern of BWO variability agrees with sediment core reconstructions and indicates that the shallower TAN1106-28 (Pacific) sediment core represents the upper circulation limb (before ventilation), whereas the deeper TN057-14PC and PS2498-1 cores (South Atlantic) sit in the lower circulation limb (after ventilation).The results are integrated in Chapter five which concludes that the Southern Ocean plays an important role in the rapid climate changes of MIS-3. This thesis finds that observational evidence from the Southern Ocean supports the modified thermal bipolar seesaw hypothesis during MIS-3 AIM warming events. Additionally, results from TAN1106-28 infer that warming and southward shifted westerlies are likely drivers of reduced iron delivery to the southwest Pacific, which reduces export production and biological pump efficiency, particularly during late MIS-3 AIM warming events. In the subsurface ocean, Chapters three and four support an expansion of PDW into the Southern Ocean, and an increase in Southern Ocean convection during AIM warming events. This thesis suggests that Southern Ocean convection acts as conduit for the outgassing of carbon from the deep ocean to the atmosphere in both the Pacific and Atlantic sectors of the Southern Ocean during AIM warming events. These new findings highlight the important role that both biological and physical Southern Ocean processes play on the carbon cycle during abrupt AMOC slowdowns of the last glacial period.