Biology, geology, and paleoclimatology studies of coral reefs often rely on annual rates of corallite extension determined from cores of skeletal material recovered from scleractinian corals. To ...examine how corallite orientation within such cores may affect measured extension rates, we used high-resolution X-ray computed axial tomography (CT) to image a suite of short
Orbicella faveolata
cores from corals living in the northwest Gulf of Mexico. This nondestructive method enabled skeletal extension rates to be determined several ways: (1) along transects that cross individual slice/slab images, (2) along transects that cross both a 0.6-mm-thick CT slice and the corresponding conventional X-ray image from a 8.4-mm-thick slab of skeletal material, and (3) along transects that cross slices/slab images oriented at varied angles relative to a coral core’s longitudinal axis. Additionally, the true three-dimensional (3D) extension rates of individual corallites were determined and compared to the extension rates apparent on a two-dimensional (2D) slice/slab. Results suggest that differences between all methods are not statistically significant, confirming that the conventional extension rate methodology is suitable for studies if the core’s longitudinal axis is appropriately aligned with the growth axis of the corallites. Effects of core misalignment on extension rates measurement and the appearance of skeletal structures in 2D images are described, and the applicability of 3D imagery for investigations of coral polyp growth patterns and coral colony development is demonstrated.
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Gravity-driven chromatography columns are used in scientific, engineering, medical, and industrial fields to separate desired compounds from solutions. Running multiple columns ...simultaneously saves time and improves procedural consistency. Though column chromatography is widely used, to meet their laboratory needs many investigators must resort to designing and fabricating custom racks for holding their chromatography columns. We have created a robust column rack design, with collection vial holders, that is easily made, inexpensive to build, and may be easily adapted to fit experimental needs. The column holder can be made to hold various sizes of columns (and can be interchanged as necessary); the height of columns above collection vials can be precisely set; and the design is modular, so the rack and vial holders can be expanded to accommodate the desired numbers of columns and the numbers and sizes of vials used to collect fractions eluted from each column. Importantly, the rack is made of inexpensive, readily–available materials and the fabrication is straightforward. Here we present details of the rack’s features, a list of materials, and instructions for making it. We hope our design will help investigators who perform gravity-driven column chromatography.
The Guayas estuary is an ecologically and economically vital, large estuarine system located on the western coast of South America. A suite of ∼400 water samples was collected and analyzed to enable ...investigation of the sources of water types within this estuary, as well as environmental processes active within various portions of it. These samples were obtained at sites distributed across the major areas of the inner and outer portions of the estuary at the ends of consecutive dry (December/January) and rainy (May) seasons. At each site, Van Dorn bottles were lowered into the water from a boat (or bridge) and then triggered when they reached specified depths. When the Van Dorn bottles were brought back aboard the boat, aliquots of water from them were promptly sealed in glass containers for later analysis. These were transported to onshore laboratories where their salinities, and their oxygen (δ18O) and hydrogen (δ2H) isotopic compositions were measured. Established analytical procedures and standards were employed to obtain a robust set of resultant data. The samples, analyses, and data presented here support the associated research article, “Stable isotope evidence for the origins of waters in the Guayas estuary and Gulf of Guayaquil” 1, to which readers are referred for interpretation.
Abstract
Approximately 380,000 underway measurements of sea surface salinity, temperature, and carbon dioxide (CO
2
) in the Gulf of Mexico (GoM) were compiled from the Surface Ocean CO
2
Atlas ...(SOCAT) to provide a comprehensive observational analysis of spatiotemporal CO
2
dynamics from 1996 to 2017. An empirical orthogonal function (EOF) was used to derive the main drivers of spatial and temporal variability in the dataset. In open and coastal waters, drivers were identified as a biological component linked to riverine water, and temperature seasonality. Air-sea flux estimates indicate the GoM open (− 0.06 ± 0.45 mol C m
−2
year
−1
) and coastal (− 0.03 ± 1.83 mol C m
−2
year
−1
) ocean are approximately neutral in terms of an annual source or sink for atmospheric CO
2
. Surface water pCO
2
in the northwest and southeast GoM open ocean is increasing (1.63 ± 0.63 µatm year
−1
and 1.70 ± 0.14 µatm year
−1
, respectively) at rates comparable to those measured at long-term ocean time-series stations. The average annual increase in coastal CO
2
was 3.20 ± 1.47 µatm year
-1
for the northwestern GoM and 2.35 ± 0.82 µatm year
−1
for the west Florida Shelf. However, surface CO
2
in the central (coastal and open) GoM, which is influenced by Mississippi and Atchafalaya River outflow, remained fairly stable over this time period.
Widespread drought characterized the Heinrich 1 and Younger Dryas cold periods of the last deglaciation throughout much of Africa, causing large increases in dust emissions from the Sahara and Sahel. ...At the same time, increases in wind strength may have also contributed to dust flux, making it difficult to interpret dust records alone as reflecting changes in rainfall over the region. The Niger River has the third largest drainage basin in Africa and drains most of the Sahara and Sahel and thus preserves and propagates climatic signals. Here, we present new reconstructions of Niger Delta sea surface salinity and Niger River discharge for the last 20,000 years in order to more accurately reconstruct the onset of the Western African Monsoon system. Based on calculated δ18OSEAWATER (δ18OSW) and measured Ba/Ca ratios in planktonic foraminifera, these new records reflect changes in sub-Saharan precipitation across the Niger River Basin in West Africa and reveal that the West African Monsoon system began to intensify several thousand years after the equatorial Monsoon system in Central Africa. We also present new records of primary productivity in the Niger Delta that are related to wind-driven upwelling and show that productivity is decoupled from changes in Niger River discharge. Our results suggest that wind strength, rather than changes in monsoon moisture, was the primary driver of dust emissions from the Sahara and Sahel across the last deglaciation.
•Delayed onset of the West African Monsoon across interior West Africa.•West African Monsoon evolution decoupled from other African Monsoon systems.•Wind strength significantly influenced West African dust emissions.
Core-top samples from different ocean basins have been analyzed to refine our current understanding of the sensitivity of benthic foraminiferal calcite magnesium/calcium (Mg/Ca) to bottom water ...temperatures (BWT). Benthic foraminifera collected from Hawaii, Little Bahama Bank, Sea of Okhotsk, Gulf of California, NE Atlantic, Ceara Rise, Sierra Leone Rise, the Ontong Java Plateau, and the Southern Ocean covering a temperature range of 0.8 to 18°C were used to revise the Cibicidoides Mg/Ca-temperature calibration. The Mg/Ca–BWT relationship of three common Cibicidoides species is described by an exponential equation: Mg/Ca = 0.867 ± 0.049 exp (0.109 ± 0.007 × BWT) (stated errors are 95% CI). The temperature sensitivity is very similar to a previously published calibration. However, the revised calibration has a significantly different preexponential constant, resulting in different predicted absolute temperatures. We attribute this difference in the preexponential constant to an analytical issue of accuracy. Some genera, notably Uvigerina, show apparently lower temperature sensitivity than others, suggesting that the use of constant offsets to account for vital effects in Mg/Ca may not be appropriate. Downcore Mg/Ca reproducibility, as determined on replicate foraminiferal samples, is typically better than 0.1 mmol mol
−1 (2 S.E.). Thus, considering the errors associated with the Cibicidoides calibration and the downcore reproducibility, BWT may be estimated to within ±1°C. Application of the revised core-top Mg/Ca–BWT data to Cenozoic foraminiferal Mg/Ca suggests that seawater Mg/Ca was not more than 35% lower than today in the ice-free ocean at 50 Ma.
Milankovitch proposed that summer insolation at mid-latitudes in the Northern
Hemisphere directly causes the ice-age climate cycles. This
would imply that times of ice-sheet collapse should ...correspond to peaks in
Northern Hemisphere June insolation. But the penultimate deglaciation has
proved controversial because June insolation peaks 127 kyr ago whereas
several records of past climate suggest that change may have occurred up to
15 kyr earlier. There is a clear signature
of the penultimate deglaciation in marine oxygen-isotope records. But dating
this event, which is significantly before the 14C age range,
has not been possible. Here we date the penultimate deglaciation in a record
from the Bahamas using a new U-Th isochron technique. After the necessary
corrections for α-recoil mobility of 234U and
230Th and a small age correction for sediment mixing, the midpoint
age for the penultimate deglaciation is determined to be 135 ± 2.5 kyr
ago. This age is consistent with some coral-based sea-level estimates, but
it is difficult to reconcile with June Northern Hemisphere insolation as the
trigger for the ice-age cycles. Potential alternative driving mechanisms for
the ice-age cycles that are consistent with such an early date for the penultimate
deglaciation are either the variability of the tropical ocean-atmosphere
system or changes in atmospheric CO2 concentration controlled by
a process in the Southern Hemisphere.
Surface sediments from Little Bahama Bank (LBB ), intersecting the subtropical thermocline, were used to assess the influence of temperature on the incorporation of Mg, Sr, F, and Cd into shells of ...benthic foraminifera. Samples were obtained from twelve ☐ cores along the southern slope of LBB, covering a temperature range of 18-4.5°C between 301 and 1585 m. We studied the composition of ten calcitic and one aragonitic species, which are often used in paleochemical reconstructions. Mg/Ca ratios decrease with increasing water depth in all benthic species, both with calcitic and aragonitic mineralogy, showing a strong correlation with water temperature. Similar decrease is seen in Sr/Ca but with no correlation with temperature. None of the benthic species studied here exhibits a depth or temperature related change in F/Ca. Similar trends are observed when using an ocean-wide dataset, which includes shallow and deep core tops (300–5000 m). We suggest that temperature is the primary control on the Mg content of benthic foraminifera. Based on inorganic precipitation experiments and thermodynamic considerations, presented here, a 30–40% decrease in the Mg distribution coefficient in calcite may be expected as a result of a temperature change from 25°C to 5°C, which is about half the observed change in LBB. A calibration curve applied to
C. pachyderma data from core tops along the slope of Little Bahama Bank suggests that water temperature may be inferred from Mg/Ca ratios with an uncertainty of about ±0.8°C. Therefore, the Mg content of benthic foraminifera may provide a new, independent temperature proxy for studying shallow waters paleoceanography. The linear decrease in Sr/Ca with increasing depth is not correlated with temperature; the trend is constant from the ocean surface down to 5 km, suggesting that pressure related effects on the calcification process are a more likely explanation than post-depositional dissolution. Mg/Ca ratios in aragonitic shells of
H. elegans covary with temperature, in accord with recent observations from corals. In contrast, the Sr and F chemistry of
H. elegans is very different than that of corals and inorganically precipitated aragonites. The disparities between the elemental composition of biogenic and inorganic phases and the large intergeneric and interspecific differences observed both in planktonic and benthic foraminifera implicate temperature related physiological processes in regulating the coprecipitation of elements in foraminiferal shells. Our work demonstrates that Cd/Ca ratios of shallow calcitic species reflect the vertical distribution of nutrients; no significant influence of temperature on the partitioning of Cd into the shells was found. Our data extend the previous deep water calibration (
Boyle, 1992), thereby allowing for the reconstruction of the nutrient chemistry of shallow thermocline waters.
In order to understand the driving forces for Pleistocene climate change more fully we need to compare the timing of climate events with their possible forcing. In contrast to the last interglacial ...(marine isotope stage (MIS) 5) the timing of the penultimate interglacial (MIS 7) is poorly constrained. This study constrains its timing and structure by precise U–Th dating of high-resolution δ
18O records from aragonite-rich Bahamian slope sediments of ODP Leg 166 (Sites 1008 and 1009). The major glacial–interglacial cycles in δ
18O are distinct within these cores and some MIS 7 substages can be identified. These sediments are well suited for U–Th dating because they have uranium concentrations of up to 12 ppm and very low initial
230Th contributions with most samples showing
230Th/
232Th activity ratio of >75. U and Th concentrations and isotope ratios were measured by thermal ionisation mass spectrometry and multiple collector inductively coupled plasma mass spectrometry, with the latter providing dramatically better precision. Twenty-nine of the 41 samples measured have a δ
234U value close to modern seawater suggesting that they have experienced little diagenesis. Ages from 27 of the 41 samples were deemed reliable on the basis of both their U and their Th isotope ratios. Ages generally increase with depth, although we see a repeated section of stratigraphy in one core. Extrapolation of constant sedimentation rate through each substage suggests that the peak of MIS 7e lasted from ∼237 to 228 ka and that 7c began at 215 ka. This timing is consistent with existing low precision radiometric dates from speleothem deposits. The beginning of both these substages appears to be slightly later than in orbitally tuned timescales. The end of MIS 7 is complex, but also appears to be somewhat later than is suggested by orbitally tuned timescales, although this event is not particularly well defined in these cores.
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