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Stable isotope‐based reconstructions of past ocean salinity and hydroclimate depend on accurate, regionally constrained relationships between the stable oxygen isotopic composition of seawater (δ¹⁸O_sw) and salinity in the surface ocean. An increasing number of δ¹⁸O_swobservations suggest greater spatial variability in this relationship than previously considered, highlighting the need to reassess these relationships on a global scale. Here, we use available, paired δ¹⁸O_swand salinity data (N = 11,119) to create global interpolations of each variable. We then use a self‐organizing map, a specialized form of machine learning, to define 19 regions with unique δ¹⁸O_sw‐salinity relationships in the surface (<50 m) ocean. Inclusion of atmospheric moisture‐related variables and oceanic tracer data in additional self‐organizing map experiments indicates global surface δ¹⁸O_sw‐salinity spatial patterns are strongly forced by the atmosphere, as the SOM spatial output is highly similar despite no overlapping input data. Our approach is a useful update to the previously delimited regions, and highlights the utility of neural network pattern extraction in spatiotemporally sparse data sets.

Hydroclimatic variability in the tropical Pacific reflects large‐scale ocean‐atmosphere processes that imprint on global climate, but tropical Pacific hydroclimate variability over the last millennium remains uncertain. Here we present a multi‐proxy reconstruction from a lacustrine sediment record on Kiritimati Atoll that spans the last millennium. A lake salinity reconstruction from the δ²H values of total lipid extracts, combined with changes in sediment mineralogy and the stable isotopic composition of inorganic carbonates, indicate the presence of several distinct hydrologic regimes on Kiritimati, including a period of persistent aridity from 960–1030 CE, shorter episodes of aridity from 1030–1370 CE, and extreme aridity throughout the latter half of the last millennium, from 1370–1970 CE, that drove complete evaporation of the lake. A period of wetter, variable conditions defines recent decades. Compound‐specific lipid biomarker δ²H values indicate that shifts in microbial community structure occurred in the past, from a community dominated by photoautotrophic and chemoautotrophic organisms in more arid periods to a more metabolically diverse community in the recent period. Comparison of this record with nearby sediment records from Washington Island reveals a similar pattern of aridity through the last millennium, followed by wetter conditions in the modern period. These coherent changes in hydroclimate in the central Pacific are inconsistent with a migration of the Pacific Intertropical Convergence Zone within these latitudes. Rather, past aridity was caused by reduced El Niño‐Southern Oscillation variability, observed through the low‐pass filter of lacustrine records, or by multidecadal to multi‐century strengthening of the Pacific Walker Circulation.

Stable oxygen isotopic ratios in corals (δ¹⁸O_coral) are commonly utilized to reconstruct climate variability beyond the limit of instrumental observations. These measurements provide constraints on past seawater temperature, due to the thermodynamics of isotopic fractionation, but also past salinity, as both salinity and seawater δ¹⁸O (δ¹⁸O_sw) are similarly affected by precipitation/evaporation, advection, and other processes. We use historical observations, isotope‐enabled model simulations, and the PAGES Iso2k database to assess the potential of δ¹⁸O_coralto provide information on past salinity. Using ‘‘pseudocorals’’ to represent δ¹⁸O_coralas a function of observed or simulated temperature and salinity/δ¹⁸O_sw, we find that δ¹⁸O_swcontributes up to 89% of δ¹⁸O_coralvariability in the Western Pacific Warm Pool. Although uncertainty in the δ¹⁸O_sw‐salinity relationship influences the inferred salinity variability, corals from these sites could provide valuable δ¹⁸O_swreconstructions. Coordinated in situ monitoring of salinity and δ¹⁸O_swis vital for improving estimates of hydroclimatic change.