Antarctica's vast freshwater reservoir is crucial in understanding future sea level rise. Melting of Antarctic glaciers and the ice sheet significantly contributes to global sea level rise, impacting bottom-water formation and sea ice cover. These changes further influence subsurface warming, surface cooling, and atmospheric westerlies, ultimately affecting marine productivity and nutrient cycles. Coupled climate-ice-sheet processes can even delay anthropogenic warming in the Southern Hemisphere. Accurate climate, sea level, and carbon cycle projections necessitate reliable observational datasets of Antarctic IFD fluxes and ice volume changes. While remote sensing and direct local observations (δ¹⁸O of seawater, noble gases, etc.) provide valuable data, limitations exist in spatial and temporal coverage. The challenge lies in distinguishing IFD signals from other freshwater sources like sea ice melting and precipitation. This study proposes seawater isotope measurements (specifically δ¹⁸Osw) as a superior quantitative proxy for Antarctic IFD due to its high signal-to-noise ratio and minimal influence from other factors such as sea-ice melting and precipitation. The study uses projections of IFD forcing from a coupled climate-ice-sheet model simulation applied to iCESM to determine the detectability of anthropogenic IFD signals against natural variability, comparing the effectiveness of δ¹⁸Osw to salinity.

Previous studies have extensively documented the contribution of Antarctic ice-sheet melting to global sea level rise and its influence on Southern Ocean processes. Research has shown the impact of IFD on bottom-water formation and sea ice cover, along with its cascading effects on temperature gradients, atmospheric circulation, and marine ecosystems. Coupled climate-ice-sheet models have indicated potential delays in Southern Hemisphere warming due to these complex feedback mechanisms. Various large-scale observational campaigns using satellite and airborne technologies have generated datasets on ice mass changes and IFD, complemented by direct observations of local IFD signals using δ¹⁸O of seawater, noble gases, and other parameters. However, existing observational methods face limitations in spatial and temporal coverage. Moreover, the ability to isolate IFD signals from other freshwater sources remains a challenge.

The study utilizes the isotope-enabled Community Earth System Model (iCESM) version 1.2.2 with a 2º atmospheric and 1º oceanic resolution. Freshwater forcing data was obtained from a coupled ice-sheet-earth system model (LOVECLIP) run with three greenhouse gas emission scenarios (SSP1.1-9, SSP2.4-5, and SSP5.8-5), resulting in varying cumulative freshwater forcing by 2100 CE. Three sets of freshwater perturbation experiments were conducted in iCESM: FWF119 and FWF245, using the low and medium greenhouse gas emission scenarios from LOVECLIP respectively, focusing solely on the effect of IFD on seawater isotopes, and FWF-GHG585, incorporating both IFD and increasing greenhouse gas concentrations from the high emission scenario. A paired experiment, FWF0-GHG585, was run with the same forcing as FWF-GHG585 but without the isotopic signature of IFD (δ¹⁸Oice = 0‰) to isolate the IFD source effect. The spatial pattern of future Antarctic IFD (2001-2100 CE) was obtained from LOVECLIP, using an Empirical Orthogonal Function analysis to distribute the annual freshwater forcing spatially. The isotopic composition of Antarctic IFD (δ¹⁸Oice) was determined by analyzing ice core data from NCEI, yielding a weighted mean of -32‰. A budget-based approach, using monthly data from the control experiment, was employed to determine the contribution of precipitation, liquid run-off, and sea ice to Southern Ocean δ¹⁸Osw and salinity. The Time of Emergence (ToE) of the IFD signal was determined using a 4σ threshold based on the standard deviation of annual mean SSS and δ¹⁸Osw from a 100-year control experiment. The modeled natural variability in salinity and sea ice was evaluated against the Ocean Reanalysis System 5 (ORAS5) dataset.

The study's key findings highlight the superior performance of δ¹⁸Osw compared to salinity in detecting anthropogenic IFD. The anthropogenic IFD signal in δ¹⁸Osw emerges significantly earlier (around 2020 CE in the Ross Sea) than the salinity signal (three decades later). The higher signal-to-noise ratio of δ¹⁸Osw, coupled with the minimal influence of sea ice changes on isotope values, contributes to its effectiveness. The Ross Sea is identified as an ideal sampling area for detecting IFD signals due to the early emergence and measurability of the signal within the analytical precision of current laser spectroscopy. Spatial analysis revealed earlier ToE in the Ross Sea-Amundsen Sea-Antarctic Peninsula region compared to the Weddell Sea. While other climate effects (increased precipitation, sea ice reduction, surface warming) can influence δ¹⁸Osw, their contribution is minor compared to the IFD-induced anomalies. Paired experiments (FWF-GHG585 and FWF0-GHG585) demonstrated that IFD accounts for a substantial proportion of δ¹⁸Osw changes (up to 83% by the end of the century). The ice-sheet influence is traceable to the subsurface ocean, notably in the Antarctic Intermediate Water, largely attributed to the IFD source effect. The study underscores the importance of subsurface δ¹⁸Osw data in conjunction with surface measurements. The combination of various tracers with δ¹⁸Osw can further enhance the reconstruction of IFD trends. In the FWF0-GHG585 scenario (climate effect only), the scatter in the salinity/δ¹⁸Osw space remains consistent with natural variability until mid-21st century, with climate effects accounting for less than 17% of the net change in δ¹⁸Osw by 2076-2080 CE. However, this analysis omits the impact of iceberg melting and sub-ice-shelf melting, which need further investigation in coupled climate-ice-sheet models.

The findings strongly support the use of δ¹⁸Osw measurements as a valuable tool for early detection of anthropogenic IFD from the Antarctic ice-sheet. The earlier emergence of the signal in δ¹⁸Osw compared to salinity underscores the importance of incorporating isotopic data in sea level rise projections. The identified ideal sampling areas (Ross Sea sector) provide crucial guidance for monitoring programs. Although the study utilizes IFD data from a coupled climate-ice-sheet model of intermediate complexity, the relatively small influence of other climate effects on δ¹⁸Osw suggests robustness in the main findings. The study's limitations concerning the absence of fully interactive ice-sheet and iceberg models within iCESM are acknowledged, emphasizing the need for future research using more advanced models that incorporate these aspects.

This study demonstrates the effectiveness of seawater oxygen isotopes (δ¹⁸Osw) as an early warning system for detecting Antarctic ice-sheet freshwater discharge. The Ross Sea sector is highlighted as a prime location for monitoring efforts. Future research should focus on using advanced coupled climate-ice-sheet models to refine projections and address limitations related to iceberg melting and sub-ice-shelf melting. Integrating δ¹⁸Osw measurements with other tracers will enhance the accuracy of IFD estimations and improve sea level rise projections.

The study's primary limitation is the use of IFD data from a coupled climate-ice-sheet model of intermediate complexity (LOVECLIP), which has limitations in resolution and complexity compared to fully coupled high-resolution models. The model does not include an interactive ice-sheet model, iceberg melting effects, or subsurface melting from sub-ice-shelf cavities. These omissions could potentially affect the accuracy of the IFD projections and influence the interpretation of results. Further research with fully coupled, higher-resolution models is needed to fully explore these aspects and to account for uncertainties related to the IFD scenarios, the δ¹⁸O source value of the Antarctic IFD, and the representation of natural variability in salinity, sea ice and δ¹⁸Osw.