Robert C Thunell

University of South Carolina | USC · Department of Earth and Ocean Sciences

The particles in the sea have an organic or inorganic origin, and sediment towards deep waters and the bottom; factor that maintains life in the dark ocean and process that sequesters C from the hydrosphere and atmosphere. In the Cariaco Basin site with high biological productivity and deep anoxic waters, an array of sediment traps was maintained f...

Climate conditions and instantaneous depositional events can influence the relative contribution of sediments from terrestrial and marine environments and ultimately the quantity and composition of carbon buried in the sediment record. Here, we analyze the elemental, isotopic, and organic geochemical composition of marine sediments to identify terr...

The trace element composition of planktic foraminifera shells is influenced by both environmental and biological factors (‘vital effects’). As trace elements in individual foraminifera shells are increasingly used as paleoceanographic tools, understanding how trace element ratios vary between individuals, among species, and in response to high freq...

The removal of exchangeable oxygen from diatom opal prior to δ¹⁸Odiatom analysis is a crucial first step before analyzing frustule oxygen isotopes for paleoceanographic applications. We present the results of experiments that quantify the temperature‐dependent reactivity of biogenic silica with water under laboratory conditions. We demonstrate that...

A chemoautotrophy maximum is present in many anoxic basins at the sulfidic layer's upper boundary, but the factors controlling this feature are poorly understood. In 13 of 31 cruises to the Cariaco Basin, particulate organic carbon (POC) was enriched in ¹³C (δ¹³CPOC as high as −16‰) within the oxic/sulfidic transition compared to photic zone values...

The ratio of boron to calcium (B/Ca) in a subset of foraminifera has been shown to covary with seawater carbonate chemistry, making this geochemical signature a promising proxy for carbon cycle science. Some studies suggest complications with the B/Ca proxy in photosymbiont‐bearing planktonic foraminifera, while relatively few studies have investig...

Oceanic uptake of CO2 can mitigate climate change, but also results in global ocean acidification. Ocean acidification-related changes to the marine carbonate system can disturb ecosystems and hinder calcification by some organisms. Here, we use the calcification response of planktonic foraminifera as a tool to reconstruct the progression of ocean...

We have determined total organic carbon‐mass accumulation rates (TOC‐MARs) and δ¹⁵N values in Marine Isotope Stages (MIS) 13–10 sediments from two oligotrophic parts of the western North Atlantic Ocean—Ocean Drilling Program (ODP) Site 1063 on the Bermuda Rise and Site 1058 on the Blake Outer Ridge. Both TOC‐MARs and δ¹⁵N values vary significantly...

Lithogenic sediment input to the Cariaco Basin on the eastern Venezuelan shelf is controlled by small mountainous rivers (SMRs). The Cariaco Basin is also an area of high phytoplankton productivity as a result of strong Trade Wind‐driven coastal upwelling. Characterizing the sources that supply particulate organic carbon (POC) to the deep Cariaco B...

In this Letter, ‘δ18C’ should have been ‘δ13C’ in Fig. 3b, and the x axis should extend to 50 kyr rather than 40 kyr. This figure has been corrected online.

Oxygen minimum zones (OMZs) are predicted to expand in the near future due to continued global warming, with largest declines in oxygen occurring within the strong tropical OMZs. In this study, shifts in the position and intensity of the OMZ in the Eastern Tropical North Pacific (ETNP) were examined using three ²¹⁰Pb-dated sediment cores collected...

Seasonal and interannual variability in dinoflagellate cyst production were assessed using a 12.5 year-long sediment trap time series from the Cariaco Basin (southern Caribbean Sea). This study constitutes the longest such time series published to date, providing robust patterns of variability for individual dinoflagellate cyst taxa as well as for...

Increased storage of carbon in the oceans has been proposed as a mechanism to explain lower concentrations of atmospheric carbon dioxide during ice ages; however, unequivocal signatures of this storage have not been found¹. In seawater, the dissolved gases oxygen and carbon dioxide are linked via the production and decay of organic material, with r...

We present a 2.5-year-long sediment trap record of dinoflagellate cyst production in the Cariaco Basin, off Venezuela (southern Caribbean Sea). The site lies under the influence of wind-driven, seasonal upwelling which promotes high levels of primary productivity during boreal winter and spring. Changes in dinoflagellate cyst production is document...

The CARIACO (Carbon Retention in a Colored Ocean) Ocean Time-Series Program station, located at 10.50°N, 64.66°W, observed biogeochemical and ecological processes in the Cariaco Basin of the southwestern Caribbean Sea from November 1995 to January 2017. The program completed 232 monthly core cruises, 40 sediment trap deployment cruises, and 40 micr...

The concentration of boron in marine carbonates (B/Ca ratio) has been proposed to covary with seawater carbonate chemistry. However, a number of studies have evaluated the B/Ca proxy in planktonic foraminifera and found that boron incorporation may not be a straightforward process. The majority of previous research has been dedicated to tropical sp...

We present a 2.5 year-long sediment trap record of dinoflagellate cyst production in the Cariaco Basin, off Venezuela (southern Caribbean Sea). The site lies under the influence of wind-driven, seasonal upwelling which promotes high levels of primary productivity during boreal winter and spring. Changes in dinoflagellate cyst production is document...

A study was conducted to characterize the variability of dissolved inorganic carbon concentration and speciation in surface waters (upper 100m) of the southern Caribbean Sea in the Cariaco Basin, located off Venezuela. The spatial distribution of total CO2 (TCO2), partial pressure of CO2 (pCO2), and the saturation state with respect to aragonite (Ω...

To test whether low latitude shallow shelf deposits can provide high resolution paleoclimatic records, we utilized two cores from the Holocene sedimentary fill of incised valleys on the Sunda Shelf off Sarawak, Malaysia. We developed a new sea surface temperature (SST) record based on planktonic foraminiferal Mg/Ca for the last 7200 years. This rec...

The deep ocean is most likely the primary source of the radiocarbon-depleted CO2 released to the atmosphere during the last deglaciation. While there are well-documented millennial scale Δ14C changes during the most recent deglaciation, most marine records lack the resolution needed to identify more rapid ventilation events. Furthermore, potential...

Supplementary Figure and Supplementary Tables

To explore relationships between water column hydrography and foraminiferal geochemistry in the Eastern Tropical North Pacific, we present and Mg/Ca records from three species of planktonic foraminifera, Globigerinoides ruber, Globigerina bulloides, and Globorotalia menardii, collected from a sediment trap mooring maintained in the Gulf of Tehuante...

Planktonic foraminiferal calcification intensity, reflected by shell wall thickness, has been hypothesized to covary with the carbonate chemistry of seawater. Here we use both sediment trap and box core samples from the Santa Barbara Basin to evaluate the relationship between the calcification intensity of the planktonic foraminifera species Globig...

Oxygen minimum zones (OMZs), located below highly productive marine regions, are sites of microbially-mediated denitrification and biogeochemical cycling that have global significance. The intensity of OMZs fluctuate naturally, however, the degree of these fluctuations and a comprehensive understanding of the factors that drive these fluctuations o...

Organic matter (OM) burial in marine sediments is a potentially important control on global climate and the long-term redox state of the earth’s surface. Still, we have only a limited understanding of the processes that stabilize OM and facilitate its preservation in the geologic record. Abiotic reactions with (poly)sulfides can enhance the preserv...

Nutrient composition plays an important role in plankton speciation and therefore marine biological production. In regions dominated by upwelling, the composition and magnitude of nutrients upwelled into surface waters not only influences surface food web dynamics, but also the composition and magnitude of particles that reach deep waters, are remi...

Understanding changes to the marine nitrogen cycle on millennial and shorter timescales can help determine the influence of rapid climate change on the fixed N pool and its sources and sinks. Rapid changes in denitrification have been observed in the eastern tropical north Pacific (ETNP) and Arabian Sea; however, millennial-scale δ15N records in re...

The East Asian Monsoon (EAM) delivers vital rainfall to heavily populated, agriculturally dependent regions in Southeast Asia. Changes in EAM strength and duration influence precipitation and wind patterns, continental runoff, sea circulation, and nutrient transport in the South China Sea (SCS) and surrounding terrain. The SCS has been the focus of...

Climate warming is expected to reduce oxygen (O2) supply to the ocean and expand its oxygen minimum zones (OMZs). We reconstructed variations in the extent of North Pacific anoxia since 1850 using a geochemical proxy for denitrification (δ15N) from multiple sediment cores. Increasing δ15N since ~1990 records an expansion of anoxia, consistent with...

We collected high-resolution time series of δ 15 N from bulk organic matter (Fig. S1) at 3 sediment core sites within and near the anoxic zone of the ETNP: the Pescadero Slope (24 o 17'N, 108 o 12'W; 600 m bottom depth), Soledad Basin (25 o 13'N, 112 o 43'W; 540 m) off the coast of Baja Alternations in sediment color and the presence of mm-scale co...

In large swaths of the ocean, primary production by diatoms may be limited by the availability of silica, which in turn limits the biological uptake of carbon dioxide. The burial of biogenic silica in the form of opal is the main sink of marine silicon. Opal burial occurs in equal parts in iron-limited open-ocean provinces and upwelling margins, es...

The CARIACO Ocean Time Series program has made monthly measurements of oxygen, nutrients, and carbon system parameters (ΣCO2, alkalinity, pH) in the Cariaco Basin since 1996. At the same time, sediment traps have collected settling particles at four to five depths ranging from 150 to 1,200 m. The depth of the transition from oxic to anoxic conditio...

[1] We present a high-resolution Mg/Ca reconstruction of tropical Atlantic sea surface temperatures (SSTs) spanning the last 2000 years using seasonally representative foraminifera from the Cariaco Basin. The range of summer/fall SST over this interval is restricted to 1.5°C, while winter/spring SST varies by 4.5°C over the same time period suggest...

The East Asian Monsoon (EAM), driven by differential heating between the continent and adjacent ocean, is a critical element of global atmospheric circulation and strongly influences wind patterns, precipitation, sea circulation, continental runoff, and nutrient transport to the South China Sea and surrounding land regions. Changes in EAM strength...

Ten years of monthly observations of the δ15N of sinking particulate nitrogen (δ15N-PN (in ‰ versus atmospheric N2)=[(15N/14N)sample/(15N/14N)standard)-1]1000) in the Cariaco Basin, Venezuela, confirm that the basin's bottom sediments store information about nitrogen dynamics related to seasonal and interannual variability in regional surface ocean...

Over much of the ocean’s surface, productivity and growth are limited by a scarcity of bioavailable nitrogen. Sedimentary δ15N records spanning the last deglaciation suggest marked shifts in the nitrogen cycle during this time, but the quantification of these changes has been hindered by the complexity of nitrogen isotope cycling. Here we present a...

Biweekly sediment trap samples and concurrent hydrographic measurements collected between March 2005 and October 2008 from the Cariaco Basin, Venezuela, are used to assess the relationship between [CO32−] and the area densities (ρA) of two species of planktonic foraminifera (Globigerinoides ruber (pink) and Globigerinoides sacculifer). Calcificatio...

The oxygen isotope compositions of seven species of planktonic foraminifera (Globigerinoides ruber (pink), Globigerina bulloides, Globorotalia menardii, Globigerinoides sacculifer, Orbulina universa, Globorotalia crassaformis, and Neogloboquadrina dutertrei) were measured from biweekly sediment trap samples collected in Cariaco Basin, Venezuela bet...

Time‐series datasets of surface‐water biogenic silica (bSiO2) and deep‐water bSiO2 export in the Santa Barbara Channel (SBC) allow for elucidation of the important scales of spatial and temporal variability and an evaluation of potential controlling mechanisms. Changes in local upwelling strength is the most important factor controlling seasonal va...

a b s t r a c t We examined the variability of sea surface carbon dioxide fugacity (fCO 2sea) and its relation to temperature at the Cariaco Basin ocean time-series location (10130 0 N, 64140 0 W) for the period from 1996 through 2008. Periods of warm (positive) and cold (negative) anomalies at the station were related to variability in coastal upw...

Excess nitrogen gas (N₂^xs) was measured in samples collected at six locations in the eastern and western sub-basins of the Cariaco Basin, Venezuela, in September 2008 (non-upwelling conditions) and March 2009 (upwelling conditions). During both sampling periods, N₂^xs concentrations were below detection in...

The Cariaco Basin is a 1400-m-deep depression approximately 160 km long by 70 km wide located off the central Venezuelan coast . It is connected to the Atlantic Ocean by a sill ~100-m-deep, and two slightly deeper channels that breech it; Canal Centinela (146-m-deep) and Canal de la Tortuge (135-m-deep). High surface production rates and restricted...

Biweekly sediment trap samples and concurrent hydrographic measurements collected between March 2005 and October 2008 from the Cariaco Basin, Venezuela, are used to assess the relationship between [CO3]2- and the area densities (ho A) of two species of planktonic foraminifera (Globigerinoides ruber (pink) and Globigerinoides sacculifer). Calcificat...

The Cariaco Basin, a silled, permanently anoxic basin on the continental shelf of Venezuela with a dynamic chemocline (-240-350 m), has been subject of > 20 years of oceanographic observation and sediment trap studies. We evaluated UK'37 and the TEX86 temperature proxies using sinking particles collected in shallow sediment trap samples at 275 m (T...

Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record. However, bulk sedimentary nitrogen isotope ratios, which can be determined routinely and at minimal cost, may be altered during burial and early sedimentary diagenesis, particularly outside of continental margin settings. The causes...

Over the last few decades, rising greenhouse gas emissions have promoted poleward expansion of the large-scale atmospheric Hadley circulation that dominates the Tropics, thereby affecting behavior of the Intertropical Convergence Zone (ITCZ) and North Atlantic Oscillation (NAO). Expression of these changes in tropical marine ecosystems is poorly un...

Surface-tethered particle interceptor traps (PITs) were deployed at 50 and 100 m (1–3 days) on ten occasions in the Cariaco Basin between March 2007 and November 2009 to measure the settling fluxes of biogenic particles at 50 m (the base of the euphotic zone—Ez) and 100 m. Fluxes at these two depths were compared to concurrent fluxes estimated with...

We have determined organic carbon concentrations and isotopic compositions and benthic foraminiferal assemblages in sediments deposited between ˜500 and 340 ka at ODP Site 1063 on the northeastern flank of the Bermuda Rise. This time interval includes Marine Isotope Stage (MIS) 11, a particularly warm and long interglacial that was similar to today...

The patterns and forcing mechanisms of climate variability on decadal to centennial time scales represent a major void in our current understanding of Earth's climate system. Furthermore, the response of the low latitudes to abrupt climate change is also not well understood, as most high-resolution paleoclimate studies are from midlatitudes and hig...

Excess nitrogen gas (excess N2) was measured in samples collected at six locations in the eastern and western sub-basins of the Cariaco Basin, Venezuela, in September 2008 (non-upwelling conditions) and March 2009 (upwelling conditions). During both sampling periods, excess N2 concentrations were below detection in surface waters, increasing to ~22...

The composition and bioavailability of particulate P influence marine biological community production on both modern and geologic time-scales, and continental margins play a critical role in the supply, modification, and storage of particulate P. This study examined particulate P cycling in the Santa Barbara Basin (SBB) off the coast of southern Ca...

Iron is considered to play a large role in the cycling of Si in Fe-limited regions of the ocean, but little is known about its role in Si biogeochemistry outside these areas. Here, we present published sediment trap data, new nutrient profiles and high resolution sedimentary records (Si isotopes, Biogenic silica%, N% and C%) from the Gulf of Califo...

Centennial-scale records of sea-surface temperature and opal composition spanning the Last Glacial Maximum and Termination 1 (circa 25-6 ka) are presented here from Guaymas Basin in the Gulf of California. Through the application of two organic geochemistry proxies, the U37K' index and the TEX86H index, we present evidence for rapid, stepped change...

A sediment density flow was observed in the eastern Cariaco Basin during September 2008. Evidence suggests that this flow was likely triggered by a magnitude 5.2 earthquake that occurred on August 11, 2008, with an epicenter located at 10.51°N, 64.17°W (off the city of Cumaná, Venezuela). Elevated suspended sediments near the bottom were observed a...

Iron is considered to play a large role in the cycling of Si in Fe-limited regions of the ocean, but little is known about its role in Si biogeochemistry outside these areas. Here, we present published sediment trap data, new nutrient profiles and high resolution sedimentary records (Si isotopes, Biogenic silica%, N% and C%) from the Gulf of Califo...

Centennial-scale records of sea-surface temperature and opal composition spanning the Last Glacial Maximum and Termination 1 (circa 25-6 ka) are presented here from Guaymas Basin in the Gulf of California. Through the application of two organic geochemistry proxies, the UK37' index and the TEX86H index, we present evidence for rapid, stepped change...