Fossil Corals Research Articles

Corals Reveal an Unprecedented Decrease of Arabian Sea Upwelling During the Current Warming Era

AbstractUpwelling in the Arabian Sea driven by the Indian summer monsoon pumps deep, cold, and eutrophic seawater to the sea surface every summer. The Indian summer monsoon and the Arabian Sea upwelling were expected to intensify with global warming, following the hypothesis that the Eurasian landmass would warm faster than the Indian Ocean. Contrary to expectations, the northern Indian Ocean currently warms faster than the Indian subcontinent. A weakening of the Indian summer monsoon circulation is reported, which possibly weakens the Arabian Sea upwelling. However, a lack of observations limits understanding of current and historical changes of the Arabian Sea upwelling. Here, we reconstruct the Arabian Sea upwelling over the past millennium using modern and fossil corals. Our coral records show that the Arabian Sea upwelling intensity was very stable over the last millennium and unprecedentedly declines at present. Our finding implies anthropogenic forcing likely weakens the Arabian Sea upwelling.

Diverse nature of ubiquitous microborings in Cenomanian corals (Saxonian Cretaceous Basin, Germany)

Upper Cenomanian rocky shore conglomerates exposed in the abandoned Ratssteinbruch quarry in Dresden (Saxonian Cretaceous Basin, Eastern Germany) contain numerous small coral colonies. The skeletons are commonly encrusted with thin ferruginous microbial crusts. Skeletal elements, especially radial elements (septa), contain abundant microborings filled with iron oxyhydroxides. Natural casts of microborings were studied under SEM. Two categories of microborings (2–12 μm in diameter) were distinguished in respect of their time of production. Type 1 microborings occur in the inner part of the colonies and are typically distributed more or less along the septa in the direction of the coral growth. This type is represented by Ichnoreticulina elegans (most common traces; produced by chlorophyte green alga), Scolecia filosa (traces of cyanobacteria), and much more rarely by Conchocelichnus seilacheri (traces of red algae). They were produced during coral life (in vivo), and provide insight into the very poorly recognised skeleton microbiome of fossil corals. Chlorophyte alga Ostreobium quekettii – the most common microendolith in the skeletons of living modern corals – produces I. elegans, which dominates the Type 1 microborings. Type 2 microborings include I. elegans, S. filosa, Scolecia serrata (made by bacteria) and undetermined microborings. They occur directly below the microbial crusts coating the entire colony, or below thin ferruginous films coating the surfaces of skeletal elements. Microborings are distributed randomly or are more or less perpendicular to skeleton surfaces, demonstrating that Type 2 microborings were evidently made by microendoliths after coral death (post-mortem), when skeletal elements were exposed.

Morphological traits of reef corals predict extinction risk but not conservation status

AbstractAimThe current assessment of extinction risk in reef corals by the International Union for Conservation of Nature (IUCN) has been criticized, because coral life‐history traits associated with resilience are not reflected in the conservation status. We aimed to carry out a quantitative assessment of the link between reef coral traits and species extinction risk informed by extinctions of reef corals observed in the fossil record.LocationGlobal.Time periodPlio‐Pleistocene and present day.Major taxa studiedScleractinian reef corals.MethodsWe used morphological traits, phylogenetic information and evidence of extinction during the Plio‐Pleistocene to predict the extinction risk of contemporary reef corals. Our model was trained using 138 Caribbean fossil coral species and an automatic machine learning algorithm. We then used this model to predict the extinction risk of 674 modern coral species.ResultsModel validation confirmed 77% accuracy in predicting extinction risk of fossil corals. Extinction risk predicted by our model showed a near‐random (57%) match with the IUCN conservation status. Our model also suggested that corals in the Least Concern or Near Threatened categories might be at higher risk of extinction than currently believed.Main conclusionsMorphological traits of fossil corals linked to their extinction risk in the Plio‐Pleistocene Caribbean Sea are known to reflect the vulnerability of extant corals. However, the results from our fossil‐calibrated model do not match the IUCN assessment of reef corals, with increased overall extinction risk. This does not necessarily indicate near‐future extinction risk, because fossil extinctions are spread over thousands of years. However, we show the applicability of using fossil data to inform the extinction risk of modern corals and recommend that future assessments of extinction risk of reef corals should consider incorporating the relationship between morphological traits and resilience, calibrated by fossil data, to maximize the utility of the extinction risk assessment.

Fossil Reefs Reveal Temporally Distinct Late Holocene Lagoonal Reef Shutdown Episodes at Kiritimati Island, Central Pacific

AbstractAn extremely rare example of well‐preserved emergent Holocene fossil reefs exists at Kiritimati Island, central Pacific. Fossil reefs are rich geological archives of paleoenvironmental change. The first paleoecological surveys of two fossil reefs are presented, revealing high coral cover (40–50%) and low diversity (6 genera). Fossil coral ages suggest reefs exhibited disparity in the timing of reef development (4,113 and 1,915 cal yBP) and ecological surveys show different coral compositions (Acropora or Porites dominant), between reefs. Results constrain two discrete episodes of reef shutdown (at 2,905 and 1,705 cal yBP) as lagoonal reefs thrived, and subsequently died off, through the late Holocene. Shifts in physio‐chemical conditions associated with reduced lagoon flushing following storm‐driven changes in atoll rim morphology are argued as the driver for the staged reef die‐off. The findings have implications for interpreting past and future eco‐morphological change on atolls, given projected increases in storminess with climate change.

Impacts of Coral Growth on Geochemistry: Lessons From the Galápagos Islands

AbstractCoral geochemical climate reconstructions can extend our knowledge of global climate variability and trends over time scales longer than those of instrumental data. However, such reconstructions can be biased by coral growth and skeletal architecture, such as growth troughs, off‐axis corallite orientation, and changing growth direction. This study quantifies the impact of skeletal architecture and growth on geochemistry using measurements of coral skeletal density, extension rate, and calcification rate, and uses these metrics to improve paleoclimate reconstructions. We present paired geochemistry‐density records at Wolf Island, Galápagos, from three Porites lobata corals: two new paired density and geochemistry records from one fossil coral, and new density data from two previously published modern geochemistry records. We categorize each sampling transect used in this record by the quality of its orientation with respect to skeletal architecture. We observe relationships between geochemistry and density that are not detected using extension or calcification rate alone. These density‐geochemistry relationships likely reflect both the response of coral growth to environmental conditions and the nonclimatic impact of skeletal architecture on geochemistry in suboptimal sampling transects. Correlations of density with Sr/Ca, Ba/Ca, and Mg/Ca are consistent with the Rayleigh fractionation model of trace element incorporation into coral skeletons. Removing transects with suboptimal skeletal architecture increases mean reconstructed SST closer to instrumental mean SST, and lowers errors of reconstruction by up to 20%. These results demonstrate the usefulness of coral density data for assessing skeletal architecture and growth when generating coral paleoclimate records.

Significant Intensification of Eastern Pacific ENSO reflected in Gal pagos Corals since the Pre-Industrial Era: A Synthesis of New and Published Fossil Coral Cores from the Tropical Eastern and Central Pacific Ocean

Significant Intensification of Eastern Pacific ENSO reflected in Gal pagos Corals since the Pre-Industrial Era: A Synthesis of New and Published Fossil Coral Cores from the Tropical Eastern and Central Pacific Ocean

Modern, archaeological, and paleontological DNA analysis of a human-harvested marine gastropod (Strombus pugilis) from Caribbean Panama.

Although protocols exist for the recovery of ancient DNA from land snail and marine bivalve shells, marine conch shells have yet to be studied from a paleogenomic perspective. We first present reference assemblies for both a 623.7Mbp nuclear genome and a 15.4kbp mitochondrial genome for Strombus pugilis, the West Indian fighting conch. We next detail a method to extract and sequence DNA from conch shells and apply it to conch from Bocas del Toro, Panama across three time periods: recently-eaten and discarded (n=3), Late Holocene (984-1258 before present [BP]) archaeological midden (n=5), and mid-Holocene (5711-7187 BP) paleontological fossil coral reef (n=5). These results are compared to control DNA extracted from live-caught tissue and fresh shells (n=5). Using high-throughput sequencing, we were able to obtain S.pugilis nuclear sequence reads from shells across all age periods: up to 92.5 thousand filtered reads per sample in live-caught shell material, 4.57 thousand for modern discarded shells, 12.1 thousand reads for archaeological shells, and 114 reads in paleontological shells. We confirmed authenticity of the ancient DNA recovered from the archaeological and paleontological shells based on 5.7× higher average frequency of deamination-driven misincorporations and 15% shorter average read lengths compared to the modern shells. Reads also mapped to the S.pugilis mitochondrial genome for all but the paleontological shells, with consistent ratios of mitochondrial to nuclear mapped reads across sample types. Our methods can be applied to diverse archaeological sites to facilitate reconstructions of the long-term impacts of human behaviour on mollusc evolutionary biology.

Microstructural characteristics of the stony coral genus Acropora useful to coral reef paleoecology and modern conservation.

Identification of fossil corals is often limited due to poor preservation of external skeleton morphology, especially in the genus Acropora which is widespread across the Indo‐Pacific. Based on skeleton characteristics from thin section, we here develop a link between the internal skeleton structure and external morphology. Ten characteristics were summarized to distinguish Acropora and five related genera, including the type and differentiation of corallites, the skeleton nature of corallites (septa, columellae, dissepiments, wall), and calcification centers within septa. Acropora is distinctive for its dimorphic corallites: axial and radial. Isopora is similar to Acropora but possess more than a single axial corallites. Montipora and Astreopora (family Acroporidae) have monomorphic corallites and a synapticular ring wall, with clustered calcification center in the former and medial lines in the latter. Pocillopora and Porties are classified by distinctive dissepiments, columellae and septa. These microstructural skeleton characteristics were effective in the genus identification of fossil corals from drilled cores in the South China Sea. Eighteen detailed characteristics (ten of axial corallites, four of radial corallites, and four of coenosteum) were used in the Acropora species classification. The axial corallites size and structure (including corallite diameter, synapticular rings, and septa), the septa of radial corallites, and the arrangement of coenosteum were critical indicators for species identification. This identification guide can help paleoenvironmental and paleoecological analyses and modern coral reef conservation and restoration.

A Rapid Cooling Event Over the Western Pacific Region During the Middle Bronze Age

AbstractClimate change in the mid‐to‐late Holocene transition is very important for predicting future climate trends and understanding the relationship between abrupt climate change and the development of past human civilization. In this study, Sr/Ca ratios and δ18O records with the annual resolution extracted from four fossil corals that were growing during the Middle Bronze Age Cold Epoch (MBACE) were used to reconstruct sea surface temperature (SST) and seawater δ18O (δ18Osw) in the South China Sea (SCS) during the mid‐to‐late Holocene transition. The results indicate that the SCS experienced a rapid cooling and wetting event during the period of ∼3,500–3,800 years BP (before present year 1950). Specifically, the average SST and δ18Osw declined rapidly by ∼3°C and ∼0.65‰, respectively, over an interval of ∼100 years from ∼3,850 years BP to ∼3,750 years BP. This rapid climate change pattern recorded in coral archives broadly agrees with those in foraminiferal and stalagmite records from adjacent land and ocean areas. Consistent with other records from the North Atlantic, this cold event in the Asia‐Western Pacific region that occurred during the MBACE and was originally identified in the North Atlantic and European regions should have occurred at the global scale, which might be caused by changes in the Asian summer monsoon linked with solar irradiance and/or the North Atlantic climate. In addition, this rapid climate change might support the occurrence and timing of the outburst flood event during the Xia Dynasty and might have led to the fall of the Xia Dynasty.

Giant palaeotsunami in Kiribati: Converging evidence from geology and oral history

AbstractFor tsunami science within Oceania, the vast Central and Western Pacific (CEWEP) is an anomalous region because of the scarcity of historical tsunami observations and the complete absence of dated palaeotsunami evidence. This paper therefore records the first dated high‐magnitude palaeotsunami event within the CEWEP region. A combination of both geological data and oral history is provided for a palaeotsunami that struck remote Makin island, northernmost of the Gilbert Islands in Kiribati, toward the end of the 16th century. A previously undocumented oral tradition of giant waves is well known to the people of Makin. Narration of this legend by the Wiin te Maneaba, traditional storyteller on Makin, provided important details supporting a tsunami hypothesis. The legend preserves credible information surrounding the giant‐wave origin of Rebua and Tokia, two prominent subaerial megaclasts of blade and block geometry that were transported 80–130 m shorewards from the reef‐edge source and deposited in sideways and inverted orientations. From available hydrodynamic flow transport equations, minimum flow velocities of 7.3–16.3 m s−1 were generated, depending on whether the reefblocks were rotated or lifted onto the reef platform. The youngest U‐Th age‐dates for fossil corals retrieved from the reefblocks give a maximum age for the palaeotsunami of circa AD 1576. Several far‐field Pacific Rim and regional possibilities exist for tsunamigenesis. These include subduction‐zone seismicity and catastrophic volcanic eruption, both of which have been linked to earlier (late 15th century) palaeotsunami events recorded elsewhere in the Pacific Islands. However, the available evidence here suggests that the ~AD 1576 Makin palaeotsunami was more likely to have been locally generated by tsunamigenic offshore submarine slope failure close to Makin's western reef, associated with the giant arcuate bight structure that characterizes the northern rim of Butaritari atoll.

Millennia-Old Coral Holobiont DNA Provides Insight into Future Adaptive Trajectories

Ancient DNA (aDNA) has been applied to evolutionary questions across a wide variety of taxa. Here, we leverage aDNA from millennia-old coral fossil fragments for the first time to gain holistic insights into a rapidly declining western Atlantic reef ecosystem. We sampled four Acropora palmata fragments (dated 4215 BCE - 1099 CE) obtained from two Florida Keys reef cores. From these samples, we established it is possible to both sequence ancient DNA (aDNA) from reef cores and place the data in the context of modern-day genetic variation. We recovered varying amounts of nuclear DNA exhibiting the characteristic signatures of aDNA from all A. palmata fragments. To describe the holobiont sensu lato, which plays a crucial role in reef health, we utilized metagenome-assembled genomes as a reference to recover a large additional proportion of ancient microbial DNA from the samples. The samples shared many common microbes with modern-day coral holobionts from the same reef, suggesting remarkable holobiont stability over time. Comparing the ancient Acropora palmata data to whole-genome sequencing data from living acroporids, we found that while slightly separate, ancient samples were most closely related to individuals of their own species. Together, these results paint a broad picture of coral holobiont change over time and lay a foundation to study historical environmental stress and evolutionary constraints. By better understanding the past genetic pressures on coral, we can gain valuable insights into the adaptive potential and trajectories of reefs under changing climatic conditions in the future.

A modern scleractinian coral with a two-component calcite–aragonite skeleton

One of the most conserved traits in the evolution of biomineralizing organisms is the taxon-specific selection of skeletal minerals. All modern scleractinian corals are thought to produce skeletons exclusively of the calcium-carbonate polymorph aragonite. Despite strong fluctuations in ocean chemistry (notably the Mg/Ca ratio), this feature is believed to be conserved throughout the coral fossil record, spanning more than 240 million years. Only one example, the Cretaceous scleractinian coral Coelosmilia (ca. 70 to 65 Ma), is thought to have produced a calcitic skeleton. Here, we report that the modern asymbiotic scleractinian coral Paraconotrochus antarcticus living in the Southern Ocean forms a two-component carbonate skeleton, with an inner structure made of high-Mg calcite and an outer structure composed of aragonite. P. antarcticus and Cretaceous Coelosmilia skeletons share a unique microstructure indicating a close phylogenetic relationship, consistent with the early divergence of P. antarcticus within the Vacatina (i.e., Robusta) clade, estimated to have occurred in the Mesozoic (ca. 116 Mya). Scleractinian corals thus join the group of marine organisms capable of forming bimineralic structures, which requires a highly controlled biomineralization mechanism; this capability dates back at least 100 My. Due to its relatively prolonged isolation, the Southern Ocean stands out as a repository for extant marine organisms with ancient traits.

First sequencing of ancient coral skeletal proteins

Here we report the first recovery, sequencing, and identification of fossil biomineral proteins from a Pleistocene fossil invertebrate, the stony coral Orbicella annularis. This fossil retains total hydrolysable amino acids of a roughly similar composition to extracts from modern O. annularis skeletons, with the amino acid data rich in Asx (Asp + Asn) and Glx (Glu + Gln) typical of invertebrate skeletal proteins. It also retains several proteins, including a highly acidic protein, also known from modern coral skeletal proteomes that we sequenced by LC–MS/MS over multiple trials in the best-preserved fossil coral specimen. A combination of degradation or amino acid racemization inhibition of trypsin digestion appears to limit greater recovery. Nevertheless, our workflow determines optimal samples for effective sequencing of fossil coral proteins, allowing comparison of modern and fossil invertebrate protein sequences, and will likely lead to further improvements of the methods. Sequencing of endogenous organic molecules in fossil invertebrate biominerals provides an ancient record of composition, potentially clarifying evolutionary changes and biotic responses to paleoenvironments.

Relative sea-level stability and the radiocarbon marine reservoir correction at Natuna Island, Indonesia, since 6400 yr BP

A high-precision relative sea level (RSL) record over the past 6400 years, reconstructed from fossil coral microatoll colonies, is reported for Natuna Island, Indonesia. The timing of 11 fossil microatolls from four sites on Natuna Island is constrained by replicate 14C and 230Th dates. We investigate the local marine reservoir correction (∆R) using the replicate dates. The two sets of dates become aligned if a Marine20 ∆R of −143 ± 176 yr, 1σ (Marine13 ∆R of 15 ± 63 yr) is assigned to the 14C dates. The distribution of microatoll ages and elevations indicates that RSL was relatively stable from 6400 to 1400 yr BP at 0.2–0.7 ± 0.4 m (2σ) higher than present, before a fall to current levels. A comparison with the predictions of a suite of glacial isostatic adjustment (GIA) models for Southeast Asia suggests refinements are needed in ice-history models to fully capture our RSL data. These new constraints on past RSL of the Sunda Shelf contribute to further calibration of GIA models in the tropics, where RSL data are presently insufficient.

Age constraints on surface deformation recorded by fossil shorelines at Cape Range, Western Australia

Abstract Laterally continuous terraces along the western flank of Cape Range, Western Australia, record both past sea-level highstands and postdepositional vertical displacement. Four distinct fossil coral reef terraces extend nearly the entire length of the slowly uplifting anticlinal structure (∼100 km), enabling documentation of the timing and degree of deformation-induced elevation contamination of past sea-level estimates from fossil shorelines. Here, we present detailed elevations of the four terraces using differential global positioning system (DGPS) and airborne light detection and ranging (LiDAR) data sets, along with new ages for the three upper terraces. Geochemical dating using strontium isotope stratigraphy techniques revealed, from highest to lowest elevation: a late Miocene reef terrace, a late Pliocene shoreline, and a prominent mid-Pleistocene reef terrace (probably associated with the marine oxygen isotope stage 33–31 interglacial), along with a broad last interglacial (Eemian) reef terrace and lagoon, which terminate at the modern shoreline. Laterally variable elevation data integrated with newly defined ages for the terraces demonstrate a gradual and continuous relative deformation in the region that spans at least the last 6.5 m.y. and constrains the emergence of the Cape Range to sometime prior to the late Miocene. This data set also shows that the most recent interglacial shoreline has undergone <1.3 m of vertical warping, suggesting minimal deformation since deposition. By tracing relative uplift rates over multiple terraces for ∼100 km of coastline, we placed constraints on maximum relative sea level (RSL) for the older terraces. Most notably, we were able to place strict maximum RSL elevations of <+34 m on the Pliocene terrace and <+16.5 m on the mid-Pleistocene terrace, with probable RSL being somewhat lower.

A Continuous Record of Central Tropical Pacific Climate Since the Midnineteenth Century Reconstructed From Fanning and Palmyra Island Corals: A Case Study in Coral Data Reanalysis

AbstractAccurate estimation of central tropical Pacific (CTP) climate variability on interannual to centennial time scales is required for robust projections of future global climate trends. Here we outline an approach that blends instrumental and coral proxy observations to yield a continuous, monthly resolved record of climate evolution in the CTP spanning the past 160 years. We concatenate coral oxygen isotope (δ18O) records from multiple living and fossil corals collected from Fanning Island (4°N, 160°W) and Palmyra Island (5°N; 162°W) located in the heart of the El Niño–Southern Oscillation. We use the regularized expectation maximization (RegEM) method to impute missing data across short gaps of 5 to 23 years within and beyond individual coral records. The resulting monthly resolved Fanning/Palmyra Island climate record spans continuously from 1863 to 2016 and provides an example of how extended time series can be built from shorter coral segments. The extended record highlights the strong trend toward warmer and wetter mean conditions in late twentieth century, in agreement with the majority of climate model hindcast simulations. The continuous reconstruction also enables a direct comparison of four exceptionally strong El Niño events (1877–1878, 1940–1941, 1997–1998, and 2015–2016). Three of these very strong El Niño events in the CTP featured a precursor warm event in the prior year and that may have favored the development of a strong El Niño event.

Tectonic Evolution of Southeast Central Asian Orogenic Belt: Evidence from Geochronological Data and Paleontology of the Early Paleozoic Deposits in Inner Mongolia

In this study, we present detrital zircon U-Pb dating and paleontological data for the newly identified Ayadeng Formation in the northern margin of the North China Block (NCB) and Xibiehe Formation (molasse) in the Bainaimiao arc belt (BAB), which could provide strong evidence indicating the affinity of the BAB and the evolution of the southeast Central Asian orogenic belt (CAOB). Zircon U-Pb data of siltstone samples and paleontological data indicate the Ayadeng Formation dates back to the Early Ordovician. Although its location is near the NCB, its zircon age spectra and paleontology share a closer affinity with those of Tarim and NE Gondwana, as the U-Pb data suggest an age range of 490–2 192 Ma (peak age=629, 788, 965 and 1 935 Ma), and similar gastropod fossils are found in Tarim and NE Gondwana. The U-Pb ages of meta-sandstone samples in the Xuniwusu Formation indicate a shared inheritance with the Ayadeng Formation (before 440 Ma), and the U-Pb ages of sandstone samples in the Xibiehe Formation are concentrated, with age peaks centered at ca. 420 Ma. Fossil corals occur in these two formations, and their age components also indicate a collisional setting. Therefore, it is speculated that the BAB drifted away from Tarim or NE Gondwana during the Ordovician and became attached to northern NCB between 440–420 Ma as an exotic terrane. During the Early Paleozoic, there may have occurred a collision between an arc and a continental block.

Late Quaternary sporadic development of Desmophyllum dianthus deep-coral populations in the southern Labrador Sea with specific attention to their 14C- and 230Th-dating

About 70 fossil and live-collected specimens of Desmophyllum dianthus were collected from steep rocky surfaces and time-averaged assemblages at their base (“coral graveyards”) in the southern Labrador Sea at depths ranging from ~1700 m (Orphan Knoll) to ~2200 m (Flemish Cap). Aside from stable isotope and mineralogical analysis, U-series and 14C age determinations were performed on the samples. Fossil corals from the coral graveyards display two principal age clusters corresponding respectively to the Holocene and Marine Isotope Stage (MIS) 5c intervals. Three samples lay outside these clusters, at ~13.5 ka (Bølling-Allerød), ~ 64 ka (MIS 3) and at ~181 ka (MIS 7a). The clusters are thought to record intervals with both i) high food availability, either through pelagic primary production in the overlying water column or more effectively in relation with particulate and dissolved organic carbon transport via an active Western Boundary Undercurrent (WBUC), and ii) high carbonate micrite advection by this current from more open North Atlantic coccolithophorid production areas, maintaining an aragonite saturation horizon (now at 2300 m) slightly deeper than the coral habitat (~ 2200 m). Attempts at estimating ventilation ages from 230Th vs 14C age offsets in Holocene samples failed. This is due to uncertainties about the initial 14C-apparent age of coral skeletons and to their high initial excesses in 230Th. This initial excess relates mostly to the scavenging of 230Th produced by the dissolved marine U by the coral-feeding organic matter carried along the WBUC trajectory. In addition, discrete diagenetic U-fluxes can be documented in fossil samples. They are well illustrated in specimens from MIS 5c age, which depict variable excesses in 234U vs 238U, but similar 230Th/238U activity ratios, pointing to a recent but fractionating redistribution of uranium in the samples. These are also documented by the development of FeMn coatings of specimens exposed at the sea floor, due to the winnowing of embedding slope sediments, with the mid- to late Holocene intensified WBUC. Such diagenetic processes are to be notably taken into consideration for the dating of pre-Holocene samples. Thus, both the initial isotopic properties of corals from this WBUC-influenced, mid-continental slope, and subsequent diagenetic effects impact their calculated ages, and blur estimates of “ventilation ages” based on their 14C vs 230Th-ages offsets.

Enhanced El Niño–Southern Oscillation Variability in Recent Decades

AbstractThe El Niño–Southern Oscillation (ENSO) represents the largest source of year‐to‐year global climate variability. While Earth system models suggest a range of possible shifts in ENSO properties under continued greenhouse gas forcing, many centuries of preindustrial climate data are required to detect a potential shift in the properties of recent ENSO extremes. Here we reconstruct the strength of ENSO variations over the last 7,000 years with a new ensemble of fossil coral oxygen isotope records from the Line Islands, located in the central equatorial Pacific. The corals document a significant decrease in ENSO variance of ~20% from 3,000 to 5,000 years ago, coinciding with changes in spring/fall precessional insolation. We find that ENSO variability over the last five decades is ~25% stronger than during the preindustrial. Our results provide empirical support for recent climate model projections showing an intensification of ENSO extremes under greenhouse forcing.

A Century of Reduced ENSO Variability During the Medieval Climate Anomaly

AbstractClimate model simulations of El Niño–Southern Oscillation (ENSO) behavior for the last millennium demonstrate interdecadal to centennial changes in ENSO variability that can arise purely from stochastic processes internal to the climate system. That said, the instrumental record of ENSO does not have the temporal coverage needed to capture the full range of natural ENSO variability observed in long, unforced climate model simulations. Here we demonstrate a probabilistic framework to quantify changes in ENSO variability via histograms and probability density functions using monthly instrumental and coral‐based sea surface temperature (SST) anomalies from 1900–2005 and 1051–1150 CE. We find that reconstructed SST anomalies from modern corals from the southwest Pacific capture changes in ENSO variability that are consistent with instrumental SST data from the central equatorial Pacific. Fossil coral records indicate 100 years of relatively lower ENSO variability during part of the Medieval Climate Anomaly. Our results demonstrate that periods of reduced ENSO variability can last a century, far longer in duration than modern observations in the instrumental record of ENSO, but consistent with results from unforced climate model simulations.