Complexity of Nearshore Strontium-to-Calcium Ratio 

Variability in a Core Sample of the Massive Coral 

Siderastrea siderea Obtained in Coral Bay, St. John, 

U.S. Virgin Islands 

U.S. Department of the Interior 

U.S. Geological Survey 

Open-File Report 2013–1092

By Christopher D. Reich, Ilsa B. Kuffner, T. Don Hickey, Jennifer M. Morrison, and Jennifer A. Flannery 
ii

U.S. Department of the Interior 

SALLY JEWELL, Secretary 

U.S. Geological Survey 

Suzette M. Kimball, Acting Director 

U.S. Geological Survey, Reston, Virginia: 2013 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod 

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its natural and living resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov 

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Suggested citation: 

Reich, C.D., Kuffner, I.B., Hickey, T.D., Morrison, J.M., and Flannery, J.A., 2013, Complexity of nearshore strontiumto-calcium ratio variability in a core sample of the massive coral Siderastrea siderea obtained in Coral Bay, St. John, 

U.S. Virgin Islands: U.S. Geological Survey Open-File Report 2013-1092, 12 p. 

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endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 
iii

Contents 

Abstract ......................................................................................................................................................................... 1

Introduction .................................................................................................................................................................... 1

Setting ........................................................................................................................................................................... 3

Methods ......................................................................................................................................................................... 4

Coral Collection .......................................................................................................................................................... 4

X-Rays and Extension Rates ..................................................................................................................................... 6

Sr/Ca Analysis ........................................................................................................................................................... 7

Results and Summary ................................................................................................................................................... 8

Acknowledgments ........................................................................................................................................................10

References Cited ..........................................................................................................................................................11

Figures 

1. Satellite image of St. John, U.S. Virgin Islands, shows location of coral cores collected in 

 March 2012 ........................................................................................................................................................ 3

2. Photographs of a U.S. Geological Survey scientist coring a Siderastrea siderea colony at the U.S. Virgin 

Islands using a hydraulic-powered drill (A) and extracting the core (B). The Harbor Point (HP-2) coral in St. 

John, Coral Bay, is shown prior to coring (C) and after coring (D) ..................................................................... 5

3. X-ray of the HP-2 coral slab. An example of a high-density corallite wall that would provide a good path for 

micro-sampling is identified by arrows ................................................................................................................ 7

4. Plot of strontium and calcium (Sr/Ca) for Path 1 (red) and Path 2 (blue) collected from HP-2. Sr/Ca shifts 

colder (higher Sr/Ca) in the zone where the brown banding occurs ................................................................... 9

5. Scanned image of the HP-2 coral slab shows drilling Path 1 and a prominent brown-band layer (A) and a 

strontium/calcium (Sr/Ca) plot of Path 2 overlain on X-ray (B) ......................................................................... 10

Tables 

1. Collection information for all corals cored in nearshore waters of St. Thomas and St. John, 

U.S. Virgin Islands .............................................................................................................................................. 6
iv

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

mile, nautical (nmi) 1.852 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

Area

square kilometer (km2) 0.3861 square mile (mi2)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32
1

Complexity of Nearshore Strontium-to-Calcium Ratio 

Variability in a Core Sample of the Massive Coral 

Siderastrea siderea Obtained in Coral Bay, St. John, 

U.S. Virgin Islands

By Christopher D. Reich, Ilsa B. Kuffner, T. Don Hickey, Jennifer M. Morrison, and Jennifer A. Flannery

Abstract

Strontium-to-calcium ratios (Sr/Ca) were measured on the skeletal matrix of a core sample from 

a colony of the massive coral Siderastrea siderea collected in Coral Bay, St. John, U.S. Virgin Islands. 

Strontium and calcium are incorporated into the coral skeleton during the precipitation of aragonite by 

the coral polyps and their ratio is highly temperature dependent. The robustness of this temperature 

dependence makes Sr/Ca a reliable proxy for sea surface temperature (SST). Details presented from the 

St. John S. siderea core indicate that terrestrial inputs of sediment and freshwater can disrupt the 

chemical balance and subsequently complicate the utility of Sr/Ca in reconstructing historical SST. An

approximately 44-year-long record of Sr/Ca shows that an annual SST signal is recorded but with an

increasing Sr/Ca trend from 1980 to present, which is likely the result of runoff from the mountainous 

terrain of St. John. The overwhelming influence of the terrestrial fingerprint on local seawater chemistry 

makes utilizing Sr/Ca as a SST proxy in nearshore environments very difficult.

Introduction

Coral reefs generally occur between 35° N and 32° S, where water temperature stays within a 

range from approximately 18 to 31 °C (Druffel, 1997; Veron, 2000; Corrège, 2006). This relatively 

narrow range in sea surface temperature (SST) tolerance allows corals to be useful as indicators of 

paleoclimate. Many of the shallow-water coral reefs (< 25-meter (m) water depth) in the Caribbean Sea 

and tropical Atlantic Ocean have been accumulating since 4,000 to 6,000 years ago when rising sea level 

flooded the shallow banks where modern-day corals are found (Lighty and others, 1978; Toscano and 

Lundberg, 1998; Balsillie and Donoghue, 2004).

Current understanding of climate variability has been limited by the short duration of 

instrumental temperature records. Since corals can live for long periods of time and grow in areas that 

are relatively consistent with respect to temperature and salinity, they are ideal for retrospectively 

extending instrumental SST records. Corals secrete skeletons composed of calcium carbonate (CaCO3), 

in the form of the mineral aragonite, through a process known as skeletogenesis (Beck and others, 1992; 

Druffel, 1997; Corrège, 2006; Lough and Cooper, 2011). During the secretion of aragonite, corals 

substitute trace elements such as barium (Ba), lead (Pb), magnesium (Mg), manganese (Mn), and 
2

strontium (Sr) in place of calcium (Ca) (Druffel, 1997; Corrège, 2006). Studies on skeletal strontium-tocalcium (Sr/Ca) suggest that the variability of Sr in the aragonite is highly dependent upon the 

temperature of seawater (Weber, 1973; Smith and others, 1979; Beck and others, 1992; Alibert and 

McCulloch, 1997; Swart and others, 2002; Corrège, 2006). The warmer the water, the less Sr is 

incorporated into the aragonite matrix; therefore, lower Sr/Ca equates to warmer SST and higher Sr/Ca 

to cooler SST (Weber, 1973; Smith and others, 1979). Corals also deposit annual high- and low-density 

bands, akin to tree rings, which are useful in age-dating the coral (Lough, 2008; Lough and Cooper, 

2011). The measurement from one high-density band to the next is the annual linear extension rate, 

which can vary from year to year due to various environmental factors, such as thermal (cold or hot) 

stress, disease, water quality, or predation (fish or boring organisms).

The application of Sr/Ca to paleothermometry is not without caveats. Modern-day technology 

and precise measurement of Sr and Ca have reduced the errors of past studies; however, there are other 

factors that can affect Sr and Ca concentration such as species-specific partition coefficients, 

metabolism, growth rate, other biological parameters known as “vital effects,” sampling techniques, and 

local seawater Sr/Ca variability (Smith and others, 1979; DeVilliers and others, 1994; Swart and others, 

2002; DeLong and others, 2011, Kuffner and others, 2012). The goal of Sr/Ca thermometry is to 

measure the elemental concentration of Sr and Ca obtained from high-resolution sampling (closely

spaced samples) of the coral skeleton and calibrate the Sr/Ca results with local surface water temperature

data so that a calibration equation can be calculated. This assumes that local seawater Sr/Ca is 

conservative and stable. The calibration equation is then applied to the entire coral Sr/Ca record to 

calculate historical SST.

Coral cores from the U.S. Virgin Islands were collected for use in retrospective studies on 

climate variability using strontium-to-calcium ratios as a proxy for SST. Collection of cores from the 

U.S. Virgin Islands (St. Thomas and St. John) provided specimens from a tropical region of the 

Caribbean Sea, which will be compared to existing results from coral cores of the same species collected 

from the subtropical region in the Florida Keys (U.S.A.). This document reports on Sr/Ca data obtained 

from a core sample from a single colony of the massive coral Siderastrea siderea collected from Harbor 

Point (Coral Bay) in eastern St. John, U.S. Virgin Islands (fig. 1).
3

Figure 1. Satellite image of St. John, U.S. Virgin Islands, shows location of coral cores collected in March 2012. 

The Siderastrea siderea HP-2 (Harbor Point) core, discussed in this report, is located between Coral Harbor 

and Coral Bay along the eastern shore of St. John. NPS, National Park Service; VICR, Virgin Islands Coral Reef 

National Monument; VIIS, Virgin Islands National Park. 

Setting 

St. John is located at approximately 18.3° N, 64.8° W, in the island arc chain of the eastern 

Caribbean Sea (fig. 1). The island of St. John encompasses an area of ~52 square kilometers and is 

mountainous with a maximum elevation of 380 m (Rankin, 2002; Hall and KellerLynn, 2010). The 

population of St. John is approximately 4,170 (2010 census), but more than 400,000 tourists visit the 

island annually (Hall and KellerLynn, 2010). The Virgin Islands National Park (VIIS), established in 

1956, covers over 60 percent (7,000 + acres) of St. John (fig. 1; http://www.nps.gov/viis/). In 2001, the 

VIIS expanded to include a 3-mile-wide belt of submerged land, and the Virgin Islands Coral Reef 

National Monument (VICR) was established (Hall and KellerLynn, 2010; http://www.nps.gov/vicr/).

The geology of the U.S. Virgin Islands is composed of the Lameshur Volcanic-Intrusive 

Complex of the Cretaceous Period (Rankin, 2002). This geologic complex consists of basalts, andesite,

and minor calcareous rocks and cherts that were uplifted and tilted as the Caribbean Plate moved from 

the western Pacific to its current position (Rankin, 2002). Though the Virgin Islands are situated within a 
4

seismically active region, recent activity has subsided, and, unlike Cuba or Puerto Rico, there is no 

evidence of uplift or fault scarps since the late Pleistocene Epoch (Rankin, 2002).

The Harbor Point (HP-2) core was collected along the eastern end of St. John, within Coral Bay, 

off a projecting land mass known as Harbor Point (fig. 1). Coral Bay is a large, multi-lobed embayment 

that is fringed by mangroves, contains fringing coral reefs, and is home to ~634 people (2010 census; 

http://www.nps.gov/viis/). Both Coral Bay, south of Harbor Point, and Coral Harbor, north of Harbor 

Point, have been subjected to increased sedimentation over the past several decades due to road 

construction, residential and commercial construction, and other anthropogenic activities (Brooks and 

others, 2007; Gray and others, 2012). Enhanced sedimentation occurs as a result of these land-use 

changes during large precipitation events, such as hurricanes. Bottom sediments transition from 

terrigenous (volcanic derived) at the head of Coral Harbor, to open marine (carbonate derived) near the 

center of Coral Bay (Brooks and others, 2007). Harbor Point is in the transition zone for the mixing of 

terrigenous and carbonate sediments.

Methods

Coral Collection

Site selection and identification of potential coral colonies for sampling occurred in September 

2011, at which time thermographs were deployed near prospective, healthy, massive corals. 

Thermographs were deployed in water depths <5 m at locations near Flat Cays in St. Thomas and near 

Leduck Island and Harbor Point in St. John.

These sites were revisited in March 2012. The largest and healthiest corals from two species, 

Siderastrea siderea and Montastraea faveolata, were identified at the prospective locations and cored 

along the primary growth axis. In addition, a single Diploria strigosa colony was cored near Steven Cay 

on the western shore of St. John (fig. 1). In total, eight coral heads were cored from the three species (see 

table 1 for core statistics and Reich and others, 2012, for further details). Corals were drilled using a 

Stanley DL07 underwater hydraulic drill and 4-inch-diameter core barrel (fig. 2A) using similar methods 

as Reich and others (2009) and Hickey and others (2012). A single core was collected from each coral 

head (fig. 2B). Cores were collected at three locations on St. John: Steven Cay, Leduck Island, and 

Harbor Point (fig. 1). This report, however, only discusses the HP-2 core collected from Harbor Point 

located in Coral Bay, St. John. The HP-2 core was collected from the massive coral S. siderea (fig. 2C,

table 1).
5

Figure 2. Photographs of a U.S. Geological Survey scientist coring a Siderastrea siderea colony at the U.S. Virgin 

Islands using a hydraulic-powered drill (A) and extracting the core (B). The Harbor Point (HP-2) coral in 

St. John, Coral Bay, is shown prior to coring (C) and after coring (D). A live plug was inserted and epoxied into 

place after the core was extracted (D). Plug diameter is 10.2 centimeters. 
6

Table 1. Collection information for all corals cored in nearshore waters of St. Thomas and St. John, U.S. Virgin 

Islands. Details for the Harbor Point (HP-2) core collected on St. John are discussed in this report.

[Dates are shown by month/day/year. USVI, U.S. Virgin Islands; N, north; W, west; cm, centimeter; m, meter]

Date 3/16/12 3/17/12 3/17/12 3/19/12 3/20/12 3/20/12 3/21/12 3/21/12

Site ID FC-1 FC-2 FC-3 LD-1 HP-1 HP-2

a SC-1 SC-2

Location Flat Cays Flat Cays Flat Cays Leduck Island

Harbor 

Point

Harbor 

Point Steven Cay

Steven 

Cay

USVI island St. Thomas St. Thomas St. Thomas St. John St. John St. John St. John St. John

Latitude (N) 18.3166 18.3162 18.3161 18.3175 18.3399 18.3399 18.3316 18.3306

Longitude (W) 64.9882 64.9883 64.9882 64.6902 64.7054 64.7054 64.807 64.8082

Total core length (cm) 77.5 60.3 100.3 35.6 165.1 92.1 80.0 54.6

Water depth (m) 4.85 4.24 4.54 3.63 2.72 3.93 3.63 0.3

Coral species S. siderea S. siderea

M. 

faveolata S. siderea M. faveolata S. siderea S. siderea

D. 

strigosa

aPresented in this report.

A short plug with live coral tissue was collected from the base of the coral head and inserted into 

the open hole where the core was extracted. This method, similar to that described by Hudson (1983), 

was implemented in order to speed up the recovery time of the coral. The short plug from the base was 

placed in the hole and epoxied into place (fig. 2D). In most situations the dense skeletons of S. siderea

and D. strigosa complicated and prevented the live tissue core plug transplant method. Since the live 

tissue plug could not be used for many of the S. siderea and D. strigosa corals, a precast cement plug 

was epoxied within the hole so that no live tissue edges were left exposed.

X-Rays and Extension Rates

Cores were sectioned along the primary growth axis into 4-millimeter-thick slabs. Multiple (~3) 

slabs were obtained because corallite walls tend to migrate over time and therefore one slab may have a 

more continuous path for geochemical sampling. Slabs were placed in deionized water and 

ultrasonicated using a Branson Sonifier with probe to remove fine-grained material. Slabs were 

allowed to completely dry prior to being X-rayed. The HP-2 slab identified for sampling was placed on a 

phosphor plate and X-rayed at 55 kilovolts and 2.5 milli-Ampere-seconds (mAs). The distance between 

the plate and the X-ray source was 79 centimeters (cm). The plate was scanned on an iCR3600+ scanner 

at 254 dots per inch (dpi) resolution (10 pixels mm-1) and processed on iCRco, Inc. software. The X-ray

(fig. 3) was exported as both 16-bit Tagged Image File Format (TIFF) and native Digital Imaging and 

Communications in Medicine (DICOM) file formats.

X-ray images were examined for high- and low-density bands. A couplet of high- and lowdensity bands (light and dark) on the X-ray is equivalent to one year of growth. Adobe Photoshop was 

used to measure the distance between couplets. An estimate of annual growth, or linear extension, rate

over the length of the coral slab was calculated (Hudson and others, 1976; Helmle and others, 2011).

Extension rate calculations are necessary to calculate a proper sampling interval for coral powder 

collection via a micro-drill. A minimum of 12 samples per year are necessary to potentially capture the 

annual seasonal cycle in Sr/Ca. However, there is a trade-off between number of samples per year and 

quantity of material; increased sampling rate per year equates to less coral material per sample for Sr/Ca 

analyses.
7

Figure 3. X-ray of the HP-2 coral slab. An example of a high-density corallite wall that would provide a good path 

for micro-sampling is identified by arrows. Note faint high- and low-density bands on the X-ray. Scale is in 

centimeters. 

Sr/Ca Analysis 

The X-ray image of the HP-2 coral slab was used to determine the appropriate path for microsampling. Generally, the best path to sample is the longest continuously growing corallite wall that 

appears as a longitudinal high-density band on the X-ray. The drilling path was traced along the corallite 

wall using Adobe Illustrator (fig. 3). The traced path was exported from Adobe Illustrator as a Drawing 

Exchange Format (DXF) file and imported into the SuperCam software operating the XYZ drill. 
8

A 1 millimeter (mm) tungsten carbide-tipped drill bit was used to sample the coral. Coral powder was 

collected along the growth axis in increments of 0.25 mm (Path 1) and 0.20 mm (Path 2) to a depth of 

1.25 mm, then placed in pre-labeled micro-centrifuge tubes. Based on X-ray-determined linear extension 

rates, approximately 12 to 16 samples per year were collected at this sampling increment. A divot in the 

skeleton was made every 20 samples for future referencing and is visible on subsequent 

X-ray images.

Sr and Ca were measured on a PerkinElmer 7300 Dual View Inductively Coupled Plasma Optical 

Emission Spectrometer (ICP-OES) at the U.S. Geological Survey St. Petersburg Coastal and Marine 

Science Center (SPCMSC). For sample analyses, ~100 micrograms (μg) of coral powder was weighed, 

placed into an ICP tube, and acidified with 2 percent nitric acid (HNO3) to an approximate concentration 

of 20 parts per million (ppm) Ca. To measure precision and correct for potential instrumental drift, an 

internal gravimetrical standard (IGS) solution was measured for Sr/Ca before and after each dissolved 

coral sample (Schrag, 1999). The average corrected IGS precision for Sr/Ca was 0.009 millimoles per 

mole (mmol/mol) (1σ, n=205). A second standard—homogenized powder from the coral Porites lutea

(PL)—was analyzed for Sr/Ca to test for any potential matrix effects. The PL standard had an average 

corrected precision of 0.019 mmol/mol (1σ, n=297).

Results and Summary

Extension rate (ER) measurements on HP-2 were difficult because the S. siderea coral collected 

from St. John deposited a very dense skeletal matrix (fig. 3). The dense composition made discerning 

annual banding (contrast between high- and low-density band couplets) challenging, even after making 

brightness and contrast adjustments to the X-ray image. Average linear extension rate, based on X-ray

measurements, for the full length of the core (92.1 cm) was 0.40 centimeters per year (cm/yr) (n=155). 

This ER dates the coral base to approximately the year 1782 (~230 years old).

In addition to measuring high- and low-density band couplets on X-rays, we measured the 

distance between the presumed annual peaks on the Sr/Ca curve. Extension rates based on X-ray

measurements for the top 12 cm ranged from 0.32 to 0.79 cm/yr with an average of 0.51 cm/yr (n=24).

At 12 cm from the top of the core, the last sample would have been collected at ~1989 

(~23 years ago). In contrast, ER based on Sr/Ca peaks, for the top 12 cm, ranged from 0.12 to 0.46 

cm/yr, with an average of 0.26 cm/yr (n=45). The sampling period, based on ER calculated from Sr/Ca 

peaks, would therefore range from 2012 to ~1968 (~44 years).

The difference in average ER (0.21 cm/yr) between the two methods is cause for concern because 

sampling rate is based on these values, where minimum sampling density is 12 samples per year and 

ideal is ~20 samples per year. The slower linear extension rates, based on the Sr/Ca measurements, 

indicate that the goal of obtaining a minimum of 12 samples per year was marginally achieved and that

by recalculating the actual average sampling rate, we only managed to sample at ~10 samples per year 

for Path 1 (0.25 mm/sample) and ~13 samples per year (0.20 mm/sample) for Path 2. Both of these 

sampling rates are at the minimal range for capturing full seasonal range in the Sr/Ca record. 

Two overlapping paths were drilled to confirm Sr/Ca results and to observe any variation in 

results from changing the sampling rate. Annual Sr/Ca cyclicity is best captured in the Sr/Ca record from 

Path 2, but its continuity over time is influenced (controlled) by external anthropogenic sources (figs. 4,

5). A typical open-ocean Sr/Ca record from S. siderea in the Dry Tortugas (Florida Keys, U.S.A.) has no 

mean shift and annual cycles are distinct and highly correlative to SST (for example, see DeLong and 

others, 2011). Path 2, with a higher sampling resolution than Path 1, shows a slightly more prominent 
9

annual cycle in Sr/Ca. Annual Sr/Ca cycles for both sampling paths indicate a cool water signal (higher 

Sr/Ca) that began in ~1980 at the start of the brown band and thereafter begin to show SST warming to 

the present (figs. 4 and 5B).

Figure 4. Plot of strontium and calcium (Sr/Ca) for Path 1 (red) and Path 2 (blue) collected from HP-2. Sr/Ca shifts

colder (higher Sr/Ca) in the zone where the brown banding occurs. Bottom right graph is an expanded view of 

the 2- to 4-centimeter interval that shows the annual cycles of Sr/Ca. The distance between annual cycles

(values in centimeters per year shown above the brackets) for Path 2 (blue) represents the linear extension rate

for HP-2.

Sr/Ca results from HP-2 show the complexity of using Sr/Ca as an SST proxy in the nearshore

environment (fig. 5). Utilization of Sr/Ca as a proxy for SST is ideal for locations possessing few 

extraneous inputs such as terrestrial discharge of sediments and freshwater (DeLong and others, 2011). 

HP-2 is located in Coral Bay, near the mouth of one of the largest catchment areas on St. John. During 

heavy rainfall events, runoff of sediment-laden water flows into Coral Harbor and into the northern 

portions of Coral Bay (Brooks and others, 2007). These inputs can influence the chemistry of the surface 

water, impacting the Sr/Ca values of the deposited aragonite skeletal matrix. In addition, sediment is also 

occasionally incorporated in the skeleton, as seen in HP-2 (fig. 5A). The brown band that occurs between 

7 and 9 cm down core was most likely the result of a severe runoff event leading to very turbid waters 

inundating the corals for several days. Assuming this scenario, the sediment would have been heavy 

enough to blanket the corals for an extended period of time but not severe enough to result in complete 
10

mortality. Over time the sediment would then have been incorporated into the skeletal matrix. Results 

from the HP-2 core indicate that Sr/Ca is a challenging proxy for retrospective SST studies in nearshore 

corals because of the complexities in surface-water chemistry due to the influence of terrestrial runoff. 

Figure 5. Scanned image of the HP-2 coral slab shows drilling Path 1 and a prominent brown-band layer (A) and a 

strontium/calcium (Sr/Ca) plot of Path 2 overlain on X-ray (B). Plotting the Sr/Ca on the X-ray shows the large 

deviation within the brown-band layer. Scale is in centimeters; millimoles per mole is shown as mmol/mol. 

Acknowledgments 

The Department of the Interior Southeast Climate Science Center and U.S. Geological Survey 

Coastal and Marine Geology Program funded this project. Many thanks to Tyler Smith, Steve 

Prosterman, Rick Nemeth, and Kevin Brown at the University of the Virgin Islands, St. Thomas, for 

logistics support and to Randy Fish and the staff at the Virgin Islands Environmental Research 

Station/Clean Islands International on St. John for lodging and access to the boat dock. This project was 

conducted under U.S. Virgin Islands Territory Government Permit #001-122. Many thanks to the 

multiple reviewers who improved this report.
11

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