Geology topics

Julie N. Richey

Publications and source records attributed to Julie N. Richey.

At least 19 recordsLinked to original sources

USGS2026

Shifting phenology in planktonic foraminifera in the Gulf of America (Mexico) with 21st century warming

We investigate trends in foraminiferal phenology using a 12-year (2008–2020), well-resolved (7 to 28-day resolution) time series of planktonic foraminiferal fluxes from a U.S. Geological Survey sediment trap in the northern Gulf of America (Gulf of Mexico; 27.5°N, 90.3°W). The total foraminiferal flux declined over this time. We used Horn-Morisita dissimilarity analysis, principal coordinate analysis, and correlation matrices to show that fall assemblages became more similar in composition to summer assemblages (July–September) over the 12 years. As a result, winter (January–March) and fall (October–December) assemblages, which dominate the total annual foraminiferal flux, became more dissimilar in composition. This pattern is consistent with multidecadal, non-uniform trends of increasing seasonal sea surface temperatures, affecting the seasonality and composition of foraminiferal communities through changing species phenology. We speculate that similar phenological changes across plankton communities might have led to trophic mismatch, causing the decline in total foraminiferal flux.

Journal of Foraminiferal Research↗

USGS2026

Synthesizing a twelve-year sediment trap time series of planktic foraminiferal flux in the Gulf of America (Mexico)

Sediment trap time series provide powerful frameworks for testing hypotheses about planktic foraminiferal assemblage composition, seasonality, and geochemical responses to environmental variability, all of which are central to improving paleoceanographic reconstructions. We present high resolution foraminiferal assemblage data from a long-running sediment trap (2008–2020) in the northern Gulf of America (Mexico). This study summarizes the species composition, seasonality, and size distribution of the fifteen most abundant species of planktic foraminifera, which account for 98% of total flux. Foraminiferal flux peaks in winter and reaches a minimum in summer, following the seasonal pattern of primary production in the northern Gulf. Winter assemblages are dominated by non-spinose taxa, whereas spinose taxa prevail during summer. Across nearly all species, average monthly test size covaries with temperature, independent of seasonal flux trends. Notably, Trilobatus sacculifer and Neogloboquadrina dutertrei show a significant decline in relative abundance and flux during 2017–2020, nearly disappearing from the assemblage.

Journal of Foraminiferal Research↗

USGS2026

Precipitation intensity increased across the South Pacific Convergence Zone in the Industrial Age

The position, strength, and variability of precipitation in the South Pacific Convergence Zone (SPCZ) remain challenging for coupled ocean-atmosphere climate models to represent, leaving future rainfall projections in this dynamically important region uncertain. Here, we present quantitative records of mean annual precipitation (MAP) spanning the past 1,000 years using sediment cores from five freshwater lakes on different islands in the western portion of the SPCZ. Precipitation reconstructions are based on the hydrogen isotope composition of the dinoflagellate biomarker dinosterol, which is inversely correlated with MAP. We show that MAP in the western tropical Pacific increased over the past ∼300 years, with a significant increase beginning in 1,820 ± 75 C.E., coincident with a warming trend in the Western Pacific Warm Pool. These results contrast with the expected drying associated with reduced Pacific Walker Circulation as global temperatures increase in many climate models. Future warming may promote more intense SPCZ precipitation.

Geophysical Research Letters↗

USGS2025

Insights from growing Globorotalia truncatulinoides and Globorotalia menardii in the laboratory

The vast majority of planktic foraminiferal culture studies have been carried out on spinose species of foraminifera, with relatively few studies on non-spinose species. We conducted a pilot study to test whether live specimens of the non-spinose planktic foraminifera, Globorotalia truncatulinoides and Globorotalia menardii , could be successfully harvested from offshore plankton tow samples in the Gulf of America (Gulf of Mexico) and kept alive in a laboratory at the US Geological Survey St. Petersburg Coastal and Marine Science Center. We collected several G. truncatulinoides specimens (n = 39) from the surface mixed-layer (0–80 meters) via vertical plankton tow in February 2020 during a sediment trap mooring recovery cruise. We collected G. menardii (n = 27) from the upper 200 meters of the water column on follow-up cruises in December 2021 and November 2022. The G. truncatulinoides specimens stayed alive in the laboratory for 8–76 days, and G. menardii for 7–29 days. All non-spinose foraminifera in this study showed a strong preference for eating marine snow aggregates from the plankton tow over Artemia nauplii. Using a combination of morphometric observations and geochemical analysis of the foraminiferal tests, we demonstrate that some specimens of both species grew new chambers while in culture, whereas other individuals added a calcite crust to the final whorl. The G. menardii were cultured in 87 Sr-labeled seawater, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to verify the presence of laboratory-grown calcite. Our results shed light on the feeding behavior and growth patterns in these two upper-ocean species of non-spinose foraminifera. This study demonstrates the feasibility of conducting laboratory culture experiments with G. truncatulinoides and G. menardii collected via plankton tow in the open ocean.

Journal of Foraminiferal Research↗

USGS2025

Snapshots of mid-to-late Holocene sea-surface temperature variability from a subtropical western Atlantic coral reef

Large-scale Holocene climate reconstructions rely heavily on extratropical proxy records. Coral-based temperature reconstructions from the tropical and subtropical oceans therefore fill a critical spatial and temporal data gap, allowing for reconstruction of seasonally resolved temperature variability. We present five new, monthly-resolved sea-surface temperature (SST) reconstructions (between 39 and 57 years in length) from 2 to 7 thousand years ago (ka) based on the strontium-to‑calcium ratio (Sr/Ca) of Orbicella faveolata corals from subtropical reefs in south Florida. Modern calibrations between O. faveolata Sr/Ca and in situ SST from the region allow us to directly compare the mean and variability of SSTs since the mid-Holocene. In contrast to the low climate variability observed in more tropical areas of the western Atlantic during the Holocene, our records from subtropical south Florida exhibit pronounced changes in mean SST and variability. Our records suggest that mid-Holocene SSTs in the Florida Keys were highly variable, with relatively cooler winters driving a cooler mean SST at ∼6.7 ka (23.7 ± 0.6°C at 6.7 ka and 25.0 ± 0.5°C at 6.6 ka), and relatively warmer summers and more variable temperatures by 5.8 ka (27.1 ± 0.4°C, seasonality of 8.7°C). We also analyzed stable oxygen isotopes in two of our corals and those data support our Sr/Ca-based estimate of climatic warming between 6.6 ka and 5.8 ka (−3.6‰ and − 3.9‰). Both winter and summer temperatures were significantly cooler than the other mid-to-late Holocene snapshots at 3.6 ka (21.2 ± 0.5°C) and SST warmed but remained highly variable at 2.6 ka (25.0 ± 0.6°C, seasonality of 7.9°C). These centennial-scale changes in climate variability potentially contributed to the regional shutdown of reef accretion by the late Holocene. Our reconstructions provide a proof-of-concept study that highlights the value of coral-based SST records from highly sensitive, subtropical locations for understanding Holocene climate on seasonal to centennial timescales.

Florida↗

USGS2025

Validation of the U37K' paleotemperature proxy in the South Brazilian Bight from core-top sediments

The paleothermometer based on the alkenone unsaturation index ( U 37 K ′ "> U37K′ ) is often used to reconstruct past sea surface temperatures (SST). In the SW Atlantic Ocean, however, a limited understanding of the seasonal and depth distribution of coccolithophores, which generates the U 37 K ′ "> U37K′ signal preserved in ocean sediments, hinders accurate estimates of past regional SSTs. We analyzed 45 core-top sediment samples from the continental shelf of the South Brazilian Bight (SBB, 23 °S to 28 °S) to assess regional spatial U 37 K ′ "> U37K′ -SST gradients and improve SST estimates. The U 37 K ′ "> U37K′ data were converted to SST using six published paleotemperature equations and compared to modern observational SST data from the World Ocean Atlas (2018) data set. Data indicate that the U 37 K ′ "> U37K′ signal is produced during the austral summer and autumn when regional episodic upwelling events occur over the inner-shelf and the nutricline shoals at the slope. Our analysis of core-top data shows that SST estimates derived from most calibration equations closely align with modern observed temperatures but are skewed toward warmer months associated with upwelling-derived nutrients. Our findings underscore the importance of considering regional and seasonal biases to improve the accuracy of paleotemperature reconstructions. Understanding the factors influencing the SBB U 37 K ′ "> U37K′ signal enables a more meaningful comparison between regional paleoceanographic studies, improving our understanding of past changes in the SW Atlantic Ocean and our ability to predict regional SST response to ongoing and future warming.

South Brazilian Bight↗

USGS2024

20th century warming in the lower Florida Keys was dominated by increasing winter temperatures

Long-lived Atlantic coral species like Orbicella faveolata are important archives of oceanographic change in shallow, marine environments like the Florida Keys. Not only can coral-based records extend for multiple centuries beyond the limits of the instrumental record, but they can also provide a more accurate representation of in situ conditions than gridded interpolated sea-surface temperature (SST) products for nearshore reef environments. We use the coral Sr/Ca paleothermometer to produce a 150-year (1830–1980 C.E.) monthly SST reconstruction from an O . faveolata colony collected in the Marquesas Keys, FL, USA. An important feature of our record is a significant 20th-century warming trend in winter SSTs. We hypothesize that the winter warming trend was driven partially by a decrease in upwelling associated cyclonic eddies spinning off the Florida Current. A long-term weakening of winter Florida Current transport over the 20th century could be responsible for decreased cyclonic eddy formation in the Florida Straits. Another feature of the record is pronounced multidecadal fluctuations of mean annual warming and cooling in the record, which correspond to Atlantic Multidecadal Variability (AMV), with the AMV lagging behind western Florida Keys temperatures by 5–11 years. Strong coherence between coral-based SST reconstructions in the western Florida Keys with broader scale Atlantic oceanographic trends over the past century suggests a common driver of regional SST variability.

Florida↗

USGS2022

Barium enrichment in the non-spinose planktic foraminifer, Globorotalia truncatulinoides

Observations of elevated barium-to-calcium ratios (Ba/Ca) in Globorotalia truncatulinoides have been attributed to contaminant phases, deep calcification depth and diagenetic processes. Here we investigate intra- and inter-test Ba/Ca variability in the non-spinose planktic foraminifer, G. truncatulinoides , from a sediment trap time series in the northern Gulf of Mexico to gain insights into the environmental influences on barium enrichment in this and other non-spinose species. We use laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to differentiate between the elemental composition of the crust and lamellar calcite in non-encrusted (<150 m calcification depth) and encrusted (>150 m calcification depth) specimens of G. truncatulinoides . We find that the Ba/Ca ratio in lamellar calcite is between two and three orders of magnitude higher (10–280 μmol/mol) than that of the crust (0–3 μmol/mol). We include seasonal water column profiles of the Ba/Ca ratio in the northern Gulf of Mexico and determine that the vertical gradient in seawater barium concentration cannot account for the intra-test Ba/Ca variations in G. truncatulinoides . We find the Ba/Ca ratio of the crust to be within the range observed in co-occurring spinose species of foraminifera (pink and white chromotypes of Globigerinoides ruber , and Orbulina universa ) while the range of Ba/Ca in lamellar calcite is consistent with co-occurring non-spinose foraminifera ( Pulleniatina obliquiloculata , Globorotalia menardii , G. tumida , and Neogloboquadrina dutertrei ). Our data are consistent with the hypothesis that G. truncatulinoides calcifies in a marine snow aggregate microenvironment that is enriched in barium relative to ambient seawater. We suggest that G. truncatulinoides crust is formed after the rhizopodia retract and the foraminifer detaches from its marine snow substrate.

Geochimica et Cosmochimica Acta↗

USGS2022

Contrasting Common Era climate and hydrology sensitivities from paired lake sediment dinosterol hydrogen isotope records in the South Pacific Convergence Zone

Hydroclimate on ‘Uvea (Wallis et Futuna) is controlled by rainfall associated with the South Pacific Convergence Zone (SPCZ), the southern hemisphere's largest precipitation feature. To extend the short observational precipitation record, the hydrogen isotopic composition of the algal lipid biomarker dinosterol (δ 2 H dinosterol ) was measured in sediment cores from two volcanic crater lakes on ‘Uvea. The modern lakes differ morphologically and chemically but both contain freshwater within the photic zone, support phytoplankton communities inclusive of dinosterol-producing dinoflagellates, and experience identical climate conditions. δ 2 H dinosterol values track lake water isotope ratios, ultimately controlled in the tropics by precipitation amount and evaporative enrichment. However, in 88-m-deep Lac Lalolalo a steadily decreasing trend in sedimentary δ 2 H dinosterol values from −227‰ around year 988 CE to modern values as low as −303‰, suggests this lake's evolution from an active volcanic setting to the present system strongly influenced δ 2 H dinosterol values. Although current hydrology and water isotope systematics may now reflect precipitation and evaporation in this lake, the interaction between these processes and large changes in basin morphology, geochemistry, and hydrology obstruct the recovery of a climate signal from Lac Lalolalo's sedimentary δ 2 H dinosterol records. This work emphasizes the importance of site replication and the use of complementary climate reconstruction tools, especially when using molecular proxies that may be sensitive to more than one environmental parameter. Contrary to its neighbor, duplicate δ 2 H dinosterol records from 23-m-deep Lac Lanutavake varied between −277‰ and −297‰ and indicate slightly drier conditions during the time-period known as the Medieval Climate Anomaly (MCA, 950–1250 CE). The δ 2 H dinosterol signal in Lac Lanutavake was muted compared to published records from ‘Upolu (Samoa) and Efate (Vanuatu) indicating that ‘Uvea's location is not as sensitive to precipitation variability at sites farther from the SPCZ central axis. Lithogenic runoff proxies combined with δ 2 H dinosterol support the interpretation of a relatively dry MCA on ‘Uvea, ‘Upolu, and Efate, potentially due to less intense precipitation, a contracted, or a more zonally oriented SPCZ.

Efate Island, Lac Lalolalo, Lac Lanoto'o, Lac Lanu↗

USGS2021

Holocene evolution of sea-surface temperature and salinity in the Gulf of Mexico

Flows into and out of the Gulf of Mexico (GoM) are integral to North Atlantic ocean circulation, and help facilitate poleward heat transport in the Western Hemisphere. The GoM also serves as a key source of moisture for much of North America. Modern patterns of sea-surface temperature (SST) and salinity in the GoM are influenced by the Loop Current, its eddy-shedding dynamics, and the ensuing interplay with coastal processes. Here we present sub-centennial-scale records of SST and stable oxygen isotope composition of seawater ( 18 O sw ; a proxy for salinity) over the past 11,700 years using planktic foraminiferal geochemistry in sediments from the Garrison Basin, northwestern GoM. We measured O and magnesium-to-calcium ratios in tests of Globigerinoides ruber (white) to generate quantitative estimates of past sea-surface conditions. Our results replicate and extend late Holocene reconstructions from the Garrison Basin, using which we then create composites of SST and 18 O sw . We find considerable centennial and millennial-scale variability in both SST and 18 O sw , although their evolution over the Holocene is distinct. Whereas mean-annual SSTs display pronounced millennial-scale variability, 18 O sw exhibits a secular trend spanning multiple millennia and points to increasing northwestern GoM surface salinity since the early Holocene. We then synthesize available Holocene records from across the GoM, and alongside the Garrison Basin composite, uncover substantial, yet regionally consistent, spatiotemporal variability. Finally, we discuss the role of the Loop Current and coastal influx of freshwater in imposing these heterogeneities. We conclude that dynamic surface-ocean changes occurred across the GoM over the Holocene.

Paleoceanography and Paleoclimatology↗

USGS2021

Remote and local drivers of Pleistocene South Asian summer monsoon precipitation: A test for future predictions

South Asian precipitation amount and extreme variability are predicted to increase due to thermodynamic effects of increased 21st-century greenhouse gases, accompanied by an increased supply of moisture from the southern hemisphere Indian Ocean. We reconstructed South Asian summer monsoon precipitation and runoff into the Bay of Bengal to assess the extent to which these factors also operated in the Pleistocene, a time of large-scale natural changes in carbon dioxide and ice volume. South Asian precipitation and runoff are strongly coherent with, and lag, atmospheric carbon dioxide changes at Earth’s orbital eccentricity, obliquity, and precession bands and are closely tied to cross-equatorial wind strength at the precession band. We find that the projected monsoon response to ongoing, rapid high-latitude ice melt and rising carbon dioxide levels is fully consistent with dynamics of the past 0.9 million years.

Science Advances↗

USGS2020

Imprint of regional oceanography on foraminifera of eastern Pacific Coral Reefs

The marginal marine environments of the eastern tropical Pacific (ETP) serve as an ideal natural laboratory to study how oceanographic and climatic variability influence coral-reef ecosystems. Reefs along the Pacific coast of Panamá span a natural gradient of nutrients, pH, and temperature as a result of stronger seasonal upwelling in the Gulf of Panamá relative to the Gulf of Chiriquí. The ecosystems are not only influenced by spatial and seasonal variations in oceanography but are affected by the climatic variability of the El Niño-Southern Oscillation (ENSO). Foraminifera can be robust indicators of ecosystem condition because the composition of their assemblages and the geochemistry of their tests can change rapidly in response to environmental variability. We studied benthic foraminifera in sediment samples collected from 3 m below mean sea level in the Gulf of Panamá and the Gulf of Chiriquí. Temperature loggers deployed from 2016 to 2019 showed that average temperatures were lower and more variable in the Gulf of Panamá due to seasonal upwelling. All sites in both gulfs were dominated by heterotrophic foraminifera, which was likely the result of nutrient enrichment due to upwelling, combined with ENSO effects. However, the Gulf of Chiriquí was characterized by higher abundances of symbiont-bearing foraminifera than the Gulf of Panamá. The orders Miliolida and Rotaliida dominated the foraminiferal assemblages in both gulfs, with Quinqueloculina and Rosalina being the most abundant genera in the two orders, respectively. Miliolids were less abundant in the Gulf of Panamá than in the Gulf of Chiriquí, whereas rotaliid densities were not significantly different between the two gulfs. Lower pH in the Gulf of Panamá as a result of upwelling may have contributed to the lower abundance of miliolids, which secrete tests of high-magnesium calcite. Geochemical analysis of tests of the symbiont-bearing miliolid Sorites marginalis revealed that foraminiferal Mg/Ca ratios were lower in the Gulf of Panamá than in the Gulf of Chiriquí. The offset in foraminiferal Mg/Ca is consistent with the lower mean annual temperature observed in the Gulf of Panamá due to stronger seasonal upwelling. Because the geochemistry and assemblages of foraminifera reflect differences in environmental conditions, they could potentially be used in tandem with coral proxies to reconstruct past environmental change and project the future of coral-reef systems within the ETP.

Gulf of Chiriqui, Gulf of Panama↗

USGS2019

PaCTS 1.0: A crowdsourced reporting standard for paleoclimate data

The progress of science is tied to the standardization of measurements, instruments, and data. This is especially true in the Big Data age, where analyzing large data volumes critically hinges on that data being standardized. Accordingly, the lack of community-sanctioned data standards in paleoclimatology has largely precluded the benefits of Big Data advances in the field. Building upon recent efforts to standardize the format and terminology of paleoclimate data, this article describes the Paleoclimate Community reporTing Standard (PaCTS), a crowdsourced reporting standard for such data. PaCTS captures which information should be included when reporting paleoclimate data, with the goal of maximizing the re-use value of paleoclimate datasets, particularly for synthesis work and comparison to climate model simulations. Initiated by the LinkedEarth project, this standard elicitation process involved an international workshop in 2016, various forms of digital community engagement over the next few years, and grassroots working groups. Participants in this process identified important properties across paleoclimate archives, in addition to the reporting of uncertainties and chronologies; they also identified archive-specific properties and distinguished reporting standards for new vs. legacy datasets. This work shows that at least 135 respondents overwhelmingly support a drastic increase in the amount of metadata accompanying paleoclimate datasets. Since such goals are at odds with present practices, we discuss a transparent path towards implementing or revising these recommendations in the near future, using both bottom-up and top-down approaches.

Paleoceanography and Paleoclimatology↗

USGS2019

Considerations for Globigerinoides ruber (white and pink) paleoceanography: Comprehensive insights from a long‐running sediment trap

We present a detailed analysis of the seasonal distribution, size, morphological variability and geochemistry of co‐occurring pink and white chromotypes of Globigerinoides ruber from a high‐resolution (1–2 weeks) and long‐running sediment trap time series in the northern Gulf of Mexico. We find no difference in the seasonal flux of the two chromotypes. Although flux of G. ruber is consistently lowest in winter, the flux‐weighted signal exported to marine sediments represents mean annual conditions in the surface mixed‐layer. We observe the same morphological diversity among pink specimens of G. ruber as white. Comparison of the oxygen and carbon isotopic composition (δ 18 O and δ 13 C) of two morphotypes ( sensu stricto and sensu lato ) of pink G. rube r reveals the isotopes to be indistinguishable. The test size distribution within the population varies seasonally, with the abundance of large individuals increasing (decreasing) with increasing (decreasing) sea surface temperature (SST). We find no systematic offsets in the Mg/Ca and δ 18 O of co‐occurring pink and white G. ruber . The sediment trap data set shows that the Mg/Ca‐temperature sensitivity for both chromotypes is much lower than the canonical 9% per °C, which can likely be attributed to the secondary influence of both salinity and pH on foraminiferal Mg/Ca. Using paired Mg/Ca and δ 18 O we evaluate the performance of a suite of published equations for calculating SST, sea surface salinity (SSS) and isotopic composition of seawater (δ 18 O sw ), including a new salinity‐δ 18 O sw relationship for the northern Gulf of Mexico from water column observations.

Paleoceanography and Paleoclimatology↗

USGS2019

Reconstructing precipitation in the tropical South Pacific from dinosterol 2H/1H ratios in lake sediment

The South Pacific Convergence Zone (SPCZ) is the Southern Hemisphere’s largest precipitation feature supplying freshwater to 11 million people. Despite its significance, little is known about the location and intensity of SPCZ precipitation prior to instrumental records, hindering attempts to predict precipitation changes in a warming world. Here we use sedimentary molecular fossils to establish a tool for extending the historical record of precipitation. Freshwater lake sediments and water samples were collected from 30 lakes that span a 4.6 mm d −1 range in precipitation rates from the Global Precipitation Climatology Project (GPCP). δ 2 H lakewater values from 29 lakes ranged from −29 to +23‰ and were inversely correlated ( r = −0.51, p < 0.001) with precipitation rates, likely due to the combination of the amount of precipitation plus evaporation. δ 2 H values of the dinoflagellate sterol dinosterol in surficial sediments from 21 lakes ranged from −316‰ in the Solomon Islands to −245‰ in French Polynesia. These δ 2 H dinosterol values were significantly correlated ( r = 0.71, p < 0.001) with δ 2 H lakewater and inversely correlated ( r = −0.77, p < 0.001) with mean annual precipitation rates with a sensitivity of −12.1 ± 2.6‰ (mm d −1 ) −1 . Fractionation between dinosterol and lake water (ε dinosterol/lakewater ) decreased at the driest lake sites ( r = − 0.70, p < 0.001). The empirical relationship between δ 2 H dinosterol and GPCP rainfall, although indirect, provides a means of quantitatively reconstructing past precipitation in the SPCZ region with an uncertainty of less than 3.1 mm d −1 , which compares favorably to the 1.5 mm d −1 uncertainty for the satellite-gauge based GPCP precipitation data.

Geochimica et Cosmochimica Acta↗

USGS2018

Quantifying uncertainty in Sr/Ca-based estimates of SST from the coral Orbicella faveolata

The strontium to calcium ratio (Sr/Ca) in aragonitic skeletons of massive corals provides a proxy for sea surface temperature (SST) that can be used to reconstruct paleoclimates across decades, centuries, and, potentially, millennia. Determining the reproducibility of Sr/Ca records among contemporaneous coral colonies from the same region is critical to quantifying uncertainties associated with the Sr/Ca‐SST proxy. We evaluated both intracolony and intercolony variability in Sr/Ca using five modern colonies of Orbicella faveolata collected live from the Dry Tortugas National Park, FL. We regressed all available Sr/Ca‐SST data pairs from the five O. faveolata colonies against the Advanced Very High Resolution Radiometer gridded SST data set to produce a new Sr/Ca‐SST calibration equation (Sr/Ca = −0.049 × SST + 10.460), which we suggest can be applied to O. faveolata colonies collected throughout the Gulf of Mexico/Caribbean region. We estimated total uncertainty by calculating the root‐mean‐square of the intracolony, intercolony, and analytical error terms. Our (1σ) uncertainty estimates of 0.082 mmol/mol (1.66 °C) for subannual Sr/Ca‐SST and 0.070 mmol/mol (1.43 °C) for mean annual Sr/Ca‐SST represent conservative error terms that can be applied to individual data points in single‐colony Sr/Ca‐SST reconstructions. We illustrate how these uncertainties can be significantly reduced by generating multicolony reconstructions and/or through replication of sampling within individual coral colonies. Although the uncertainties on absolute Sr/Ca‐based SST are likely too large to allow researchers to evaluate subdecadal temperature variability, we show that the O. faveolata paleothermometer can reliably detect changes of ~2 °C across decadal timescales and ~1 °C over multidecadal timescales.

Paleoceanography and Paleoclimatology↗

USGS2018

Pronounced centennial-scale Atlantic Ocean climate variability correlated with Western Hemisphere hydroclimate

Surface-ocean circulation in the northern Atlantic Ocean influences Northern Hemisphere climate. Century-scale circulation variability in the Atlantic Ocean, however, is poorly constrained due to insufficiently-resolved paleoceanographic records. Here we present a replicated reconstruction of sea-surface temperature and salinity from a site sensitive to North Atlantic circulation in the Gulf of Mexico which reveals pronounced centennial-scale variability over the late Holocene. We find significant correlations on these timescales between salinity changes in the Atlantic, a diagnostic parameter of circulation, and widespread precipitation anomalies using three approaches: multiproxy synthesis, observational datasets, and a transient simulation. Our results demonstrate links between centennial changes in northern Atlantic surface-circulation and hydroclimate changes in the adjacent continents over the late Holocene. Notably, our findings reveal that weakened surface-circulation in the Atlantic Ocean was concomitant with well-documented rainfall anomalies in the Western Hemisphere during the Little Ice Age.

Nature Communications↗

USGS2018

A North American Hydroclimate Synthesis (NAHS) of the Common Era

This study presents a synthesis of century-scale hydroclimate variations in North America for the Common Era (last 2000 years) using new age models of previously published multiple proxy-based paleoclimate data. This North American Hydroclimate Synthesis (NAHS) examines regional hydroclimate patterns and related environmental indicators, including vegetation, lake water elevation, stream flow and runoff, cave drip rates, biological productivity, assemblages of living organisms, and salinity. Centennial-scale hydroclimate anomalies are obtained by iteratively sampling the proxy data on each of thousands of age model realizations and determining the fractions of possible time series indicating that the century-smoothed data was anomalously wet or dry relative to the 100 BCE to 1900 CE mean. Results suggest regionally asynchronous wet and dry periods over multidecadal to centennial timescales and frequent periods of extended regional drought. Most sites indicate drying during previously documented multicentennial periods of warmer Northern Hemisphere temperatures, particularly in the western U.S., central U.S., and Canada. Two widespread droughts were documented by the NAHS: from 50 BCE to 450 CE and from 800 to 1100 CE. Major hydroclimate reorganizations occurred out of sync with Northern Hemisphere temperature variations and widespread wet and dry anomalies occurred during both warm and cool periods. We present a broad assessment of paleoclimate relationships that highlights the potential influences of internal variability and external forcing and supports a prominent role for Pacific and Atlantic Ocean dynamics on century-scale continental hydroclimate.

Global and Planetary Change↗