Stewart J. Fallon

The rise of mercury since the Industrial Revolution is regarded as a global signature of industrialisation. We use lake sediment records from sub-Antarctic Macquarie and Campbell Islands spanning the period before and after 1850 Common Era (CE) to investigate how local colonial disturbance influences mercury sources and cycling. Although mercury accumulation increased 1.1- to 74-fold across all lakes following European arrival (1810 CE), isotope responses were lake specific. Our findings identify colonisation as an unrecognised driver of mercury cycling that can override the global industrialisation signature in the Southern Hemisphere. Colonisation-driven ecological changes, including the introduction of invasive rabbits altered mercury cycling through two connected processes. Firstly, vegetation disturbance and erosion increased catchment-derived mercury. Secondly, marine biological mercury cycling shifted to terrestrial biological cycling through food web reorganisation. These responses reveal that colonisation, primarily through the introduction of invasive species, was a significant driver of Anthropocene mercury cycling, alongside industrialisation.

The Tropical West Pacific is an understudied region in environmental and archaeological studies when compared to continental regions. The unique geologies, climate systems, and ecological dynamics in the Tropical West Pacific greatly limit the transferrability of palaeoenvironmental proxies established from studies outside of this region. To address this gap, we present a ~5600 yr multi-proxy record of environmental change from a marine lake in Belau (Palau in English). The Belau archipelago is located in the Tropical West Pacific, a region with links to global atmospheric, oceanic, and hydrological climate systems. Additionally, this archipelago is central to debates of early human mobility in the Tropical Pacific ~3500 to 3000 years ago. By combining high-resolution data (XRF) with low-resolution proxies (pollen, fern spore, charcoal, TOC, δ ¹³ C, TN, δ ¹⁵ N, TOC:TN) we are able to reconstruct the response of a coastal Belauan system to changing Holocene climate dynamics and identify the primary processes driving these changes. Two distinct climate signatures are identified in the multi-proxy record. Drier and warmer conditions during Phase I (~ 5600 to 3250 cal yr BP, 689–400 cm) are consistent with local records of a southern displacement of the Intertropical Convergence Zone (ITCZ) and regional records of long-term cooling trends in the Northern Hemisphere. Phase II (~ 3250 to − 63 cal yr BP) marks a transition to wetter and cooler conditions in Belau, associated with the northward migration of the ITCZ and intensification of El Niño Southern Oscillation (ENSO). The Phase II transition coincides with a shift in catchment vegetation, as mangroves retreat and the rainforest expands, and evidence of rising local sea level. A brief vegetation disturbance event ~1250 to 500 cal yr BP does not align with documented changes in regional climate systems, suggesting that this was a localised signature. Currently it is unclear if an enrichment in bulk sediment δ ¹⁵ N values at ~1250 cal yr BP was driven by rising sea levels or local ecological dynamics and future studies should seek to clairfy the drivers of this signal.

The timing, rate, and magnitude of rapid sea-level rise during Meltwater Pulse 1B (MWP-1B, ~11.45–11.1 ka) remain controversial. Robust constraints on past MWPs are crucial to future predictions of global ice sheet instability. Using 154 new and existing U/Th and calibrated ¹⁴C-AMS dates from coral, algae, and microbialites recovered during Integrated Ocean Drilling Program Expedition 325, this study reconstructs reef development and relative sea-level (RSL) rise on the Great Barrier Reef (GBR). We identify 107 in situ RSL index points while refining estimates of vertical accretion and paleowater depth. Results show RSL rise during MWP-1B did not exceed 10.2–7.7 m or rates of 30–23 mm/yr, and was likely less. The GBR did not drown, indicating resilience to MWP-1B. These findings are more consistent with Tahiti and other Pacific records and do not support the Barbados record of MWP-1B as an abrupt step in global sea level, with a magnitude > 11 m.

Marine shells incorporate oxygen isotope signatures during growth, creating valuable records of seawater temperature and marine oxygen isotopic compositions. Secondary ion mass spectrometry (SIMS) measures these compositions in situ at finer

Marine shells incorporate oxygen isotope signatures during growth, creating valuable records of seawater temperature and marine oxygen isotopic compositions. Secondary ion mass spectrometry (SIMS) measures these compositions in situ at finer length‐scales than traditional stable isotope analyses. However, determining oxygen isotope ratios in aragonite, the most common shell mineral, is hampered by a lack of ideal reference materials, limiting the accuracy of SIMS‐based seawater temperature reconstructions. Here, we tested the capability of SIMS to produce seawater temperature reconstructions despite the matrix calibration challenges associated with aragonite. We cultured Anadara trapezia bivalves at four controlled seawater temperatures (13–28°C) and used strontium labeling to mark the start of the temperature‐controlled shell increment, allowing for more spatially precise SIMS analysis. An improved matrix calibration was developed to ensure more accurate bio‐aragonite analyses that addressed matrix differences between the pure abiotic reference materials and the bio‐aragonite samples with intricate mineral‐organic architectures and distinct minor and trace element compositions. We regressed SIMS‐IRMS biases of abiotic and biogenic aragonites that account for their systematic differences in major, minor, and trace elements, allowing for more accurate SIMS analyses of the temperature‐controlled shell increment. The thorough matrix calibration allowed us to provide a SIMS‐based seawater‐corrected oxygen isotope thermometer of T(°C) = 23.05 ± 0.36 − 4.48 · (δ¹⁸O_aragonite [‰ VPDB] − δ¹⁸O_seawater [‰ VSMOW] ± 0.25) and 10³lnα_{aragonite‐seawater} = (17.78 ± 0.88) · 10³/T (K) − (29.44 ± 2.40) that agrees with existing aragonitic IRMS‐based thermometer relationships and improves the applicability of SIMS‐based paleo‐environmental reconstructions of marine bio‐aragonites.

We compare depth and temporal distributions of sub‐fossil assemblages of two cold‐water scleractinian corals on seamounts in the Southwest Pacific to help define the temporal variations of water mass properties in the Southern Ocean (SO) during deglaciation. Peaks in the deep‐water abundance of the two species complement one another, with Desmophyllum dianthus peaking around the Antarctic Cold Reversal (ACR), and Solenosmilia variabilis briefly during the late Heinrich Stadial 1 (HS1) and during the Younger Dryas (YD). Environmental tolerances of the two species and the geochemistry of S. variabilis carbonate skeletons suggest that their secular distributions reflect complementary effects of temperature (higher at Antarctic Intermediate Water/Upper Circumpolar Deep Water depths during the YD and late HS1) and surface productivity (lower during the YD and HS1). Higher temperatures at depth we interpret as evidence of increased Zonal West Wind (ZWW)‐driven Ekman pumping during the late HS1 and YD, whereas coeval low surface production reflects poleward expansion of sub‐tropical water masses as a result of correlated poleward shifts of the ZWW belt and the Intertropical Convergence Zone. More broadly, a continuous deep coral population in the southwest Pacific that spans two species and three deglacial periods (HS1, ACR and the YD) and an early Holocene shift in coral distribution from deeper to shallower habitats appear to reflect large‐scale changes during deglaciation in SO temperature profiles and productivity.

The Little Ice Age (LIA) was one of the strongest climate perturbations in the last millennium, with widespread albeit asynchronous periods of cooling globally¹. Its effects on oceanic properties and thermohaline circulation have been questioned, however². We provide evidence of changes in the properties of Antarctic Intermediate Water (AAIW) over the last millennium that are synchronous with LIA cooling events and that appear to derive proximately from variations in the proportion of Upper Circumpolar Deep Water (UCDW) in the AAIW source mixture, and ultimately from changes in the strength and position of the zonal west winds over the Southern Ocean (SO). Export of AAIW from the SO influences climate and biogeochemical cycles globally by transporting heat, freshwater, nutrients and carbon to lower latitudes^3-5, implying widespread impacts of its LIA-correlated variability. AAIW tracking of the LIA ceased in the late 1800s, which we suggest marked the onset of over-riding impacts of anthropogenic climate change on SO circulation.