Abstract Coral resilience is shaped by trophodynamic flexibility – the balance between photosymbiont-derived energy and feeding-based heterotrophy – yet quantifying this balance across taxa and through time remains difficult. Nitrogen (N) isotopes are a powerful tool to investigate trophic strategies, but their use requires information on local isotopic baselines. Here, we introduce the biomineral-bound N isotopes of crustose coralline algae (CCA) as an archive that closely tracks the N isotopic composition of the reef nitrate supply, making it a proxy for a reef’s isotopic baseline. Coupled N isotope measurements of co-occurring CCA, symbiont-bearing, and symbiont-barren corals further enable us to quantify a coral’s “trophic enrichment factor,” reflecting the efficiency of the internal N recycling between the coral and its photosymbionts. From this framework, we derive a Reliance on Symbionts Index (RSI) that captures taxonomic and regional variation in mixotrophy, enabling reconstruction of coral trophodynamics in modern and fossil reef systems.

Abstract Long-term, high-frequency temperature records are critical for evaluating the vulnerability of marine ecosystems to ongoing ocean warming. Commonly used paleoceanographic archives, such as corals and bivalves, face regional and ecological constraints. Rhodoliths, free-living coralline red algal nodules, are established and globally accessible alternatives, yet their complex three-dimensional growth structures hinder the construction of continuous age models. Here we present a workflow combining staining-based growth calibration, semi-automated increment detection using micro-computed tomography, and dynamic time warping to assemble a daily chronology spanning 133 days (March–July) from seven fruticose thalli of a single rhodolith from the central Red Sea. The resulting multi-thallus chronology, combined with a multi-element temperature proxy (Magnesium-to-Strontium), yields closer agreement with in-situ logger temperatures (R² = 0.91, RMSE = 0.63 °C) than individual thalli or single-element proxies, establishing a proof-of-concept framework for daily-resolved paleotemperature reconstructions from tropical rhodoliths.

The marine nitrogen (N) cycle plays a central role in regulating ocean productivity, ecosystem structure, and biogeochemical feedbacks on climate. However, our understanding of historical changes in the marine N cycle is limited by the short time span and spotty spatial coverage of direct observations. This study explores the use of crustose coralline algae (CCA) as a novel proxy for reconstructing past changes in the marine N cycle. We use the biomineral-bound nitrogen isotopic composition of tropical CCA (δ15NCCA) and compare it to the δ¹⁵N of subsurface nitrate (δ15N Nitrate) and corals (δ15Ncoral). Our results show that CCA reliably track subsurface nitrate δ¹⁵N and, therefore, can be used to reconstruct past changes in the N cycle. Additionally, anchoring δ15N coral values to co-occurring CCA allows us to isolate the reliance on photosymbionts in coral species, revealing taxonomic and regional variation in trophic strategies. Therefore, δ15N CCA also provides a tool to investigate coral trophodynamics in the modern ocean as well as in the historical record.

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.

Greenland is particularly vulnerable to ongoing anthropogenic climate change and observational data document recent rapid mass loss of many of the Greenland Ice Sheet (GIS) glaciers. Mass loss of the GIS represents a major contributor to global sea level rise, but uncertainties in future projections are large. A recent acceleration in mass loss has been observed, with 2012 and 2018/19 record years documented by direct observations. However, estimates of melt variability and glacier runoff remain uncertain before the satellite era and the influence on surface ocean waters is unclear. In general, available observational records from high latitudes are sparse and short. Models require high-resolution data of past variability to resolve how fast the GIS reacts to warming.Past climate can be reconstructed from natural proxy archives. In high latitudes, however, most proxy time series utilised to date come from indirect land-based proxies. Calcified coralline algae are important shallow-marine calcifiers that grow attached to the seafloor and have emerged as subannual-resolution climate recorders for the extratropics. By analyzing long-lived coralline algae from Disko Bay, West-Greenland, in close proximity to Jakobshavn Glacier, we address this data gap. Jakobshavn Glacier is one of the largest glaciers in Greenland and the single largest source of mass loss from the GIS over the last 20 years. Sclerochronological analysis and ultra-high-resolution laser ablation ICP-MS data from calcified coralline algae (Clathromorphum compactum) provide seasonally-resolved records that capture the impact of surface temperature warming and glacier runoff on coastal Arctic environments. Algal Ba/Ca ratios track past glacier-derived meltwater input to the ocean surface layer and we report an unprecedented nonlinear increase in Jakobshavn glacier runoff into Disko Bay in the last 20 years. Our chronology from southern Disko Bay sites shows a distinct increasing trend from the early 2000s, recording the acceleration of GIS glacier mass loss and matching recent years of record amounts of ice loss in satellite data. The rate of increase in Ba/Ca (a runoff proxy) is unprecedented over at least the last 100 years, highlighting the rising influence of global warming on Arctic coastal ecosystems. The new algal chronology provides a long-term perspective on high-resolution variability in Jakobshavn Glacier runoff into Disko Bay, extending before observations, and confirming model data.

Sea-ice loss and increasing unpredictability have disturbed and harmed Arctic peoples and ecosystems. In addition, studies demonstrate that sea ice plays a key role in climate variability and air–sea CO2 exchanges. Sea-ice data sets provide environmental baselines, validate proxies and models, and serve in regional and temporal comparisons. Accordingly, sea-ice and sea-ice–related variables are particularly valuable in climate modeling, paleoclimatology, and ecology to document past and present environmental changes and predict future outcomes. This article provides an overview of modern, historical, and long-term proxy sea-ice data sets that cover the last millennium. We describe available Arctic sea-ice data sources, discuss each data set’s strengths and limitations, and compare multisourced Arctic sea-ice histories in different regions. We conclude with remarks on the impacts of internal forcing from natural feedbacks and oscillations versus anthropogenic impacts on sea-ice variability. We draw upon remaining uncertainties regarding causes of past sea-ice variability and advocate for continued use of multiple data sources in sea-ice reconstruction–related studies and further development of a multisourced past sea-ice data network.

Abstract Given sea ice's importance in global climate regulation, fully understanding the role of natural temperature and atmospheric patterns like the Arctic Oscillation (AO), North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation (AMO) in its variability is critical. While instrumental AMO and reliable AO records are available since the mid‐1800s and 1958, respectively, satellite sea‐ice concentration data sets start only in 1979, limiting the shared timespan to study their interplay. Growth increments of the coralline algae, Clathromorphum compactum, can provide sea‐ice proxy information for years prior to 1979. We present a seasonal 210‐year algal record from Lancaster Sound in the Canadian Arctic Archipelago capturing low frequency AMO/NAO variability and high frequency interannual AO/NAO prior to 2000. We suggest that sea‐ice variability here is strongly coupled to these large‐scale climate processes, and that sea‐ice cover was greater and the AO more negative in the early and late 19th century compared to the 20th.

Coralline algae play important ecological roles throughout the photic zone of the world’s oceans. Recent studies have shown that attached-living coralline algae can contain records of past climate variability. So far, algal-based paleo-reconstructions are mainly available from mid- to high-latitudes, while in low latitude and temperate regions only few examples exist. Here, we investigate samples from the attached-living encrusting coralline algal species Neogoniolithon hauckii (Rothpletz) R.A Townsend & Huisman [= Neogoniolithon mamillosum (Hauck) Setchell & L.R. Mason, nom illeg.] from a temperate site in the Mediterranean Sea to assess its potential as an environmental recorder. The specimens were collected at different water depths (20 and 40 m) in the Columbretes Islands (Spain). Sclerochronological analysis of sectioned samples revealed seasonal growth patterns. Mg/Ca, Li/Ca, and Ba/Ca ratios were measured in the algal skeletons using laser ablation inductively coupled mass spectrometry (LA-ICP-MS) in ultra-high resolution. We report a mean vertical extension rate of 1.1 to 1.2 mm/year (based on analysis of Mg/Ca cycles in 40 m and 20 m samples), representing the first growth rate measurement for this species. In addition, subannual banding patterns were mapped, measured, and could be linked to high frequency variability in laser-analyzed Mg/Ca ratios. Elemental ratios analyzed in Neogoniolithon hauckii were compared to in situ water temperatures measured at the water depth of sample collection. Our results show significant positive relationships between algal Mg/Ca (R=0.55) as well as Li/Ca ratios (R=0.46) and in situ measured temperature data (40 m specimen). Ba/Ca ratios show no significant correlation to temperature and may be influenced by other factors. These data suggest potential of this species for climate reconstructions in warm-temperate regions as Neogoniolithon hauckii is not only widely distributed in the Mediterranean, but also one of the few species that may be used as a temperature archive for mesophotic coralligenous assemblages that are strongly affected by the recent anthropogenic temperature rise. Further calibration studies are needed to test the element-temperature relationships on samples with longer growth records and in different settings and water depths.

The Atlantic Multidecadal Oscillation (AMO), Arctic Oscillation (AO), and related North Atlantic Oscillation (NAO) have been linked to multidecadal, decadal, and/or interannual sea-ice variability in the arctic, but their relative influences are still under evaluation. While instrumental AMO and reliable AO records are available since the mid-1800s and 1958, respectively, satellite sea-ice concentration datasets start only in 1979, limiting the shared timespan to study their interplay. Growth increments of the coralline algae, Clathromorphum compactum, can provide sea-ice proxy information for years prior to 1979. We present a seasonal 210-year algal record from Lancaster Sound in the Canadian Arctic Archipelago capturing low frequency AMO variability and high frequency interannual AO/NAO prior to 2000. We suggest that sea-ice variability here is strongly coupled to these large-scale climate processes, and that sea-ice cover was greater and the AO more negative in the early and late 19th century compared to the 20th.

Paleoclimate and paleoenvironmental reconstructions from increment-yielding archives strongly depend on precise age models. Like bivalves, corals, trees, and speleothems, the coralline alga Clathromorphum compactum produces annual growth increments and shows considerable promise as an environmental archive for arctic and subarctic regions. Though their growth increment widths correlate with temperature and sea ice cover in high Arctic regions, existing timeseries have not been crossdated. In fact, previous studies have shown a lack of inter-sample correlation in non-crossdated timeseries suggesting possible age model dating errors. Here, we use dendrochronology crossdating techniques and COFECHA software to ensure and validate synchrony between C. compactum timeseries (<141 years) from three specimens collected near Beechey Island, Nunavut, Canada. Results showed that non-crossdated timeseries constructed by four coralline red algae researchers using annual increments of the same C. compactum samples were highly variable and showcase the likelihood of dating errors in non-crossdated timeseries. Crossdating improved inter-series correlations, and correlations to sea ice-related records, suggesting that at least three crossdated timeseries are required to isolate paleoclimate signals. Our findings suggest that future reconstructions with C. compactum should employ crossdating techniques to reduce dating errors and allow for more precise climate reconstructions. Lay Abstract Long-term environmental records provide a critical baseline to examine how humans have impacted Earth’s natural climate. An important piece to consider is sea ice’s role in natural climate variability because its brightness limits warming by reflecting solar irradiation back to space. However, instrumental records of sea ice rarely extend beyond the early satellite era (late 1970s), limiting our understanding of how sea ice affects natural climate variability in the preindustrial era. A lack of historical baseline prompted the development of sea ice proxies, including the long-lived marine alga, Clathromorphum compactum. Similar to tree-rings, C. compactum produces a new mineralized layer each year, and layer thicknesses have been shown to respond to sea ice cover, making them useful to record long-term sea ice variability. However, a recent study showed that records had replicability problems, maybe due to dating mistakes. Our study applies tree-ring dating methods (dendrochronology) to match annual algal growth layers across algal specimens. Results showed that these new methods reduced dating errors, allowing for more precise past sea ice cover reconstructions.

Long-term, high-resolution measurements of environmental variability are sparse in the High Arctic. In the absence of such data, we turn to proxies recorded in the layered skeletons of the long-lived crustose coralline algae Clathromorphum compactum. Annual growth banding in this alga is dependent on several factors that include temperature, light availability, nutrients, salinity, and calcium carbonate saturation state. It has been observed that growth slows during winter as sunlight reaching the seafloor diminishes due to decreased insolation and the build-up of sea-ice, such that the relationship between sea-ice cover extent and algal growth has allowed for reconstructions of relative sea-ice variability through time. However, recent laboratory work has shown that C. compactum continue growing in complete darkness (sea-ice cover). Therefore, a more complete understanding of algal growth is necessary for the refinement of the sea-ice proxy. Here, we present the results of a ~year-long in-situ growth and environmental monitoring experiment in Arctic Bay, Nunavut, Canada (~73°N) which addresses, for the first time in situ , the gaps in our understanding of growth over an annual cycle in the High Arctic. Algal growth was assessed on a quasi-monthly basis, where specimens were subsampled to quantify monthly extension in the context of ocean temperature and light availability. By measuring extension rate through time, we observed that the algae grew on average 72 µm yr -1 , with ~54% of annual growth occurring during the sea-ice free summer months (June-September), ~25% during the winter months (November-April), and ~21% occurring during the transition months of May and October. Although winter growth slowed, we did not observe a consistent cessation of linear extension during low-or no-light months. We posit that substantial growth during the winter months at this latitude is most likely a consequence of the mobilization of stored energy (photosynthate) produced during the photosynthetically active summer months. However, we also discuss the possibility of low light-photosynthetic activity and/or dark carbon fixation, which could also facilitate extension through time. Overall, the novel growth model presented here has implications for the use of C. compactum growth for reconstructing the environment as well as for trace-element-based (typically Mg/Ca) algal chronologies.

Arctic sea ice cover has been steeply declining since the onset of satellite observations in the late 1970s. However, the available annually resolved sea ice data before this time are limited. Here, we evaluated the suitability of annual trace element (Mg/Ca) ratios and growth increments from the long‐lived annual increment‐forming benthic coralline red alga, Clathromorphum compactum, as high‐resolution sea ice cover archive. It has previously been shown that the growth of C. compactum is strongly light controlled and therefore greatly limited during the polar night and underneath sea ice cover. We compare algal data from 11 sites collected throughout the Canadian Arctic, Greenland, and Svalbard, with satellite sea ice data. Our results suggested that algal growth anomalies most often produced better correlations to sea ice concentration than Mg/Ca ratios or when averaging growth and Mg/Ca anomalies. High Arctic regions with persistently higher sea ice concentrations and shorter ice‐free seasons showed the strongest correlations between algal growth anomalies and satellite sea ice concentration over the study period (1979–2015). At sites where ice breakup took place before the return of sufficient solar irradiance, algal growth was most strongly tied to a combination of solar irradiance and other factors such as temperature, suspended sediments, phytoplankton blooms, and cloud cover. These data are the only annually resolved in situ marine proxy data known to date and are of utmost importance to gain a better understanding of the sea ice system and to project future sea ice conditions.

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 (δ 2 H 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 δ 2 H 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 δ 2 H seawater values 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.

Earth and Space Science Open Archive This work has been accepted for publication in Paleoceanography and Paleoclimatology. Version of RecordESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary. Learn more about preprints. preprintOpen AccessYou are viewing the latest version by default [v1]Suitability of the Coralline Alga Clathromorphum compactum as an Arctic Archive for Past Sea ice CoverAuthorsNatashaLeclerciDJochenHalfarSteffenHetzingerPhoebeChanWalterAdeyAlexandraTsayEricBrossierAndreasKronziDSee all authors Natasha LeclerciDCorresponding Author• Submitting AuthorUniversity of TorontoiDhttps://orcid.org/0000-0002-0570-4790view email addressThe email was not providedcopy email addressJochen HalfarUniversity of Toronto at Mississaugaview email addressThe email was not providedcopy email addressSteffen HetzingerUniversität Hamburgview email addressThe email was not providedcopy email addressPhoebe ChanUniversity of Bergen and Bjerknes Centre for Climate Researchview email addressThe email was not providedcopy email addressWalter AdeyDepartment of Botany, Smithsonian Institutionview email addressThe email was not providedcopy email addressAlexandra TsayUniversity of Genevaview email addressThe email was not providedcopy email addressEric BrossierAssociation Nord-Estview email addressThe email was not providedcopy email addressAndreas KronziDUniversitaet GoettingeniDhttps://orcid.org/0000-0001-5655-4633view email addressThe email was not providedcopy email address

The Arctic cryosphere is changing rapidly due to global warming. Northern Svalbard is a warming hotspot with a temperature rise of ~ 6 °C over the last three decades. Concurrently, modelled data suggest a marked increase in glacier runoff during recent decades in northern Svalbard, and runoff is projected to increase. However, observational data from before anthropogenic influence are sparse and the potential effects on the surface ocean are unclear. Here, we present a 200-year record of Ba/Ca ratios measured in annual increment-forming coralline algae from northern Spitsbergen as a proxy for past glacier-derived meltwater input. Our record shows a significant increasing trend in algal Ba/Ca ratios from the late-1980s onwards matching modelled regional runoff data, suggesting a drastic increase in land-based runoff. The rate of increase is unprecedented during the last two centuries and captures the impact of amplified warming on the coastal surface ocean in the high Arctic. The algal Ba/Ca runoff proxy offers an opportunity to reconstruct past land-based runoff variability in Arctic settings in high resolution, providing important data for validating and improving climate modelling studies.

Warming surface ocean temperatures combined with the continued diffusion of atmospheric CO2 into seawater have been shown to have detrimental impacts on calcareous marine organisms in tropical and temperate localities. However, greater oceanic CO2 uptake in higher latitudes may present a higher oceanic acidification risk to carbonate organisms residing in Arctic and subarctic habitats. This is especially true for crustose coralline algae that build their skeletons using high-Mg calcite, which is among the least stable and most soluble of the carbonate minerals. Here we present a century-long annually resolved growth, density, and calcification rate record from the crustose coralline alga Clathromorphum nereostratum, a dominant calcifier in Pacific Arctic and subarctic benthic communities. Specimens were collected from the Aleutian Islands, Alaska (USA), a region that has undergone a long-term decline of 0.08 ± 0.01 pH units since the late 19th century. Growth and calcification rates remain relatively stable throughout the record, but skeletal densities have declined substantially since A.D. 1983. Strong correlations to warming sea-surface temperatures indicate that temperature stress may play a significant role in influencing the ability of corallines to calcify. Decreasing algal skeletal density may offset the benefits of continued growth and calcification due to a weakening in structural integrity, which could have detrimental consequences for the diverse reef-like communities associated with algal structures in mid-to-high latitudes.

Predator loss and climate change are hallmarks of the Anthropocene yet their interactive effects are largely unknown. Here, we show that massive calcareous reefs, built slowly by the alga Clathromorphum nereostratum over centuries to millennia, are now declining because of the emerging interplay between these two processes. Such reefs, the structural base of Aleutian kelp forests, are rapidly eroding because of overgrazing by herbivores. Historical reconstructions and experiments reveal that overgrazing was initiated by the loss of sea otters, Enhydra lutris (which gave rise to herbivores capable of causing bioerosion), and then accelerated with ocean warming and acidification (which increased per capita lethal grazing by 34 to 60% compared with preindustrial times). Thus, keystone predators can mediate the ways in which climate effects emerge in nature and the pace with which they alter ecosystems.

Rising atmospheric carbon dioxide is warming Arctic seawater at a rate twice the global average due to multiple positive feedbacks. Thus, warming is disproportionately influencing data‐poor Arctic marine ecosystems. Subarctic flora are an important component of these ecosystems, along with the less biodiverse flora endemic to the Arctic. Warming will likely lead to an increasing dominance of subarctic flora that will be initially successful due to a shorter sea ice period. Benthic crustose coralline algae presently flourishes in subarctic regions where they are key substrate builders that influence community structure through provision of habitat for a variety of benthic organisms. Here we evaluate changes in the skeletal resilience of long‐lived subarctic crustose coralline alga Clathromorphum compactum to variability in seawater temperature and sea ice (the later which influences salinity and solar irradiance reaching the seafloor) across latitudes in the northwest Atlantic and Arctic Oceans. We demonstrate that average growth and calcification rate significantly decreases in C. compactum toward higher latitudes due to colder temperatures and more fresh waters. Skeletal density also declines toward higher latitudes but displays specimen‐specific variability. However, through a common growth period (1984–2001), density increased at all locations which we interpret to be a response to warmer and more well‐lit benthic environments. At the most northerly site, growth and calcification also increased with density, suggesting warming and declining Arctic sea ice in the spring may benefit this species at its upper latitudinal limits. As a result, continued warming may enhance the presence of C. compactum in Arctic regions.

The Arctic cryosphere is changing at a rapid pace due to global warming and the large-scale changes observed in the Arctic during the past decades exert a strong influence throughout the global climate system. The warming of Arctic surface air temperatures is more than twice as large as the global average over the last two decades and recent events indicate new extremes in the Arctic climate system, e.g. for the last five years Arctic annual surface air temperature exceeded that of any year since 1900 AD. Northern Spitsbergen, Svalbard, located in the High Arctic at 80°N, is a warming hotspot with an observed temperature rise of ~6°C over the last three decades indicating major global warming impacts. However, even the longest available datasets on Svalbard climatic conditions do not extend beyond the 1950s, inhibiting the study of long-term natural variability before anthropogenic influence. Ongoing climate trends strongly affect the state of both glaciers and seasonal snow in Svalbard. Modeled data suggest a marked increase in glacier runoff during recent decades in northern Svalbard. However, observational data are sparse and short and the potential effects on the surface ocean are unclear. This study focuses on the ultra-high-resolution analysis of calcified coralline algal buildups growing attached to the shallow seafloor along Arctic coastlines. Analysis of these new annually-layered climate archives is based on the long-lived encrusting coralline algae Clathromorphum compactum, providing a historic perspective on recently observed changes. Here, we present a 200-year record of past surface ocean variability from Mosselbukta, Spitsbergen, northern Svalbard. By using algal Ba/Ca ratios as a proxy for past glacier-derived meltwater input, we investigate past multi-decadal-scale fluctuations in land-based freshwater contributions to the ocean surface layer. Our records, based on multiple coralline algal specimens, show a strong and statistically significant increasing trend in algal Ba/Ca ratios from the 1990s onwards, suggesting a drastic increase in land-based runoff at Mosselbukta. The drastic rate of increase is unprecedented during the last two centuries, directly capturing the impact of amplified surface air temperature warming on coastal high Arctic surface ocean environments.