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Abstract The Gulf of Maine holds significant ecological and economic value for fisheries and communities in north-eastern North America. However, there is apprehension regarding its vulnerability to the effects of increasing atmospheric CO₂. Substantial recent warming and the inflow of low alkalinity waters into the Gulf of Maine have raised concerns about the impact of ocean acidification on resident marine calcifiers (e.g. oysters, clams, mussels). With limited seawater pH records, the natural variability and drivers of pH in this region remain unclear. To address this, we present coastal water pH proxy records using boron isotope (δ¹¹B) measurements in long-lived, annually banded, crustose coralline algae (1920–2018 CE). These records indicate seawater pH was low (~ 7.9) for much of the last century. Contrary to expectation, we also find that pH has increased (+ 0.2 pH units) over the past 40 years, despite concurrent rising atmospheric CO₂. This increase is attributed to an increased input of high alkalinity waters derived from the Gulf Stream. This delayed onset of ocean acidification is cause for concern. Once ocean circulation-driven buffering effects reach their limit, seawater pH decline may occur swiftly. This would profoundly harm shellfisheries and the broader Gulf of Maine ecosystem. 

Abstract Proxy‐based reconstructions of Neogene warm climates are a valuable data source for helping to understand what a future, warmer world may look like. Such insights are especially critical in the Arctic where the fastest rates of warming are underway and likely to continue. In this study, hydrogen isotopes of lignin‐methoxy groups (δ²H_LM) from Miocene and Pliocene sub‐fossil wood samples (N = 43) at six high‐latitude sites (73–80°N) in the Canadian Arctic Archipelago were used to estimate mean δ²H values of precipitation and temperature anomalies (ΔT) relative to present. The ΔT estimates ranged from +9.7 to +16.7°C depending on site and epoch and are corroborated by a suite of independent proxy data for most sites, and for one site (Prince Patrick Island) this study provides the first quantitative ΔT estimates. These are conservative estimates as they do not account for the more negative δ²H_seawatervalues during the Neogene. These ΔT estimates, along with independent proxy and vegetation data, depict a dramatically warmer version of the Arctic. Some of this warming was likely driven by global atmospheric change and feedbacks that are possible in the modern‐day Arctic. However, transformation of the once‐contiguous Arctic landmass into a dissected archipelago has undoubtedly changed the nature and future warming potential of the Canadian Arctic region. Investigations aimed at disentangling the relative contribution of global versus regional boundary conditions to Neogene Arctic climate warming are needed to understand the extent to which these reconstructions may foreshadow conditions in the future.