KECK GEOLOGY CONSORTIUM

PROCEEDINGS OF THE TWENTY-FIFTH ANNUAL KECK RESEARCH SYMPOSIUM IN

GEOLOGY

April 2012 Amherst College, Amherst, MA

Dr. Robert J. Varga, Editor Director, Keck Geology Consortium

Pomona College

Dr. Tekla Harms

Symposium Convenor Amherst College

Carol Morgan Keck Geology Consortium Administrative Assistant

Diane Kadyk Symposium Proceedings Layout & Design

Department of Earth & Environment Franklin & Marshall College

Keck Geology Consortium Geology Department, Pomona College

185 E. 6th St., Claremont, CA 91711 (909) 607-0651, keckgeology@pomona.edu, keckgeology.org

ISSN# 1528-7491

The Consortium Colleges The National Science Foundation ExxonMobil Corporation

KECK GEOLOGY CONSORTIUM PROCEEDINGS OF THE TWENTY-FIFTH ANNUAL KECK RESEARCH

SYMPOSIUM IN GEOLOGY ISSN# 1528-7491

April 2012

Robert J. Varga

Editor and Keck Director Pomona College

Keck Geology Consortium Pomona College

185 E 6th St., Claremont, CA 91711

Diane Kadyk Proceedings Layout & Design Franklin & Marshall College

Keck Geology Consortium Member Institutions:

Amherst College, Beloit College, Carleton College, Colgate University, The College of Wooster, The Colorado College, Franklin & Marshall College, Macalester College, Mt Holyoke College,

Oberlin College, Pomona College, Smith College, Trinity University, Union College, Washington & Lee University, Wesleyan University, Whitman College, Williams College

2011-2012 PROJECTS

TECTONIC EVOLUTION OF THE CHUGACH-PRINCE WILLIAM TERRANE, SOUTH-CENTRAL ALASKA Faculty: JOHN GARVER, Union College, Cameron Davidson, Carleton College Students: EMILY JOHNSON, Whitman College, BENJAMIN CARLSON, Union College, LUCY MINER, Macalester College, STEVEN ESPINOSA, University of Texas-El Paso, HANNAH HILBERT-WOLF, Carleton College, SARAH OLIVAS, University of Texas-El Paso. ORIGINS OF SINUOUS AND BRAIDED CHANNELS ON ASCRAEUS MONS, MARS Faculty: ANDREW DE WET, Franklin & Marshall College, JAKE BLEACHER, NASA-GSFC, BRENT GARRY, Smithsonian Students: JULIA SIGNORELLA, Franklin & Marshall College, ANDREW COLLINS, The College of Wooster, ZACHARY SCHIERL, Whitman College. TROPICAL HOLOCENE CLIMATIC INSIGHTS FROM RECORDS OF VARIABILITY IN ANDEAN PALEOGLACIERS Faculty: DONALD RODBELL, Union College, NATHAN STANSELL, Byrd Polar Research Center Students: CHRISTOPHER SEDLAK, Ohio State University, SASHA ROTHENBERG, Union College, EMMA CORONADO, St. Lawrence University, JESSICA TREANTON, Colorado College. EOCENE TECTONIC EVOLUTION OF THE TETON-ABSAROKA RANGES, WYOMING Faculty: JOHN CRADDOCK. Macalester College, DAVE MALONE. Illinois State University Students: ANDREW KELLY, Amherst College, KATHRYN SCHROEDER, Illinois State University, MAREN MATHISEN, Augustana College, ALISON MACNAMEE, Colgate University, STUART KENDERES, Western Kentucky University, BEN KRASUSHAAR INTERDISCIPLINARY STUDIES IN THE CRITICAL ZONE, BOULDER CREEK CATCHMENT, FRONT RANGE, COLORADO Faculty: DAVID DETHIER, Williams College Students: JAMES WINKLER, University of Connecticut, SARAH BEGANSKAS, Amherst College, ALEXANDRA HORNE, Mt. Holyoke College

DEPTH-RELATED PATTERNS OF BIOEROSION: ST. JOHN, U.S. VIRGIN ISLANDS Faculty: DENNY HUBBARD and KARLA PARSONS-HUBBARD, Oberlin College Students: ELIZABETH WHITCHER, Oberlin College, JOHNATHAN ROGERS, University of Wisconsin-Oshkosh, WILLIAM BENSON, Washington & Lee University, CONOR NEAL, Franklin & Marshall College, CORNELIA CLARK, Pomona College, CLAIRE McELROY, Otterbein College. THE HRAFNFJORDUR CENTRAL VOLCANO, NORTHWESTERN ICELAND Faculty: BRENNAN JORDAN, University of South Dakota, MEAGEN POLLOCK, The College of Wooster Students: KATHRYN KUMAMOTO, Williams College, EMILY CARBONE, Smith College, ERICA WINELAND-THOMSON, Colorado College, THAD STODDARD, University of South Dakota, NINA WHITNEY, Carleton College, KATHARINE, SCHLEICH, The College of Wooster. SEDIMENT DYNAMICS OF THE LOWER CONNECTICUT RIVER Faculty: SUZANNE O’CONNELL and PETER PATTON, Wesleyan University Students: MICHAEL CUTTLER, Boston College, ELIZABETH GEORGE, Washington & Lee University, JONATHON SCHNEYER, University of Massaschusetts-Amherst, TIRZAH ABBOTT, Beloit College, DANIELLE MARTIN, Wesleyan University, HANNAH BLATCHFORD, Beloit College. ANATOMY OF A MID-CRUSTAL SUTURE: PETROLOGY OF THE CENTRAL METASEDIMENTARY BELT BOUNDARY THRUST ZONE, GRENVILLE PROVINCE, ONTARIO Faculty: WILLIAM PECK, Colgate University, STEVE DUNN, Mount Holyoke College, MICHELLE MARKLEY, Mount Holyoke College Students: KENJO AGUSTSSON, California Polytechnic State University, BO MONTANYE, Colgate University, NAOMI BARSHI, Smith College, CALLIE SENDEK, Pomona College, CALVIN MAKO, University of Maine, Orono, ABIGAIL MONREAL, University of Texas-El Paso, EDWARD MARSHALL, Earlham College, NEVA FOWLER-GERACE, Oberlin College, JACQUELYNE NESBIT, Princeton University.

Funding Provided by: Keck Geology Consortium Member Institutions

The National Science Foundation Grant NSF-REU 1005122 ExxonMobil Corporation

Keck Geology Consortium: Projects 2011-2012 Short Contributions— Virgin Islands Project

DEPTH-RELATED CARBONATE CYCLING IN A MODERN REEF: ST. JOHN, U.S. VIRGIN ISLANDS Project Faculty: DENNIS K. HUBBARD, Oberlin College & KARLA PARSONS-HUBBARD, Oberlin College ESTIMATING DEPTH RELATED REEF CARBONATE PRODUCTION PATTERNS OFF ST. JOHN, USVI WILLIAM MATTHEW BENSON, Washington and Lee University Research Advisor: Lisa Greer INVESTIGATING ENDOLITHIC ALGAE PROLIFERATION USING STABLE CARBON ISOTOPES IN BOULDER STAR CORAL CORNELIA CLARKE, Pomona College Research Advisor: Robert Gaines POST-BLEACHING ENCRUSTATION HABITS IN USVI CORAL REEFS CLAIRE MCELROY, Otterbein University Research Advisor: Halard Lescinsky DEPTH-RELATED PATTERNS OF ABUNDANCE, DISTRIBUTION, AND CARBONATE PRODUCTION FOR MICROBORING ORGANISMS: ST. JOHN, US VIRGIN ISLANDS CONOR NEAL, Franklin and Marshall College Research Advisor: Roger Thomas DEPTH RELATED DISTRIBUTION AND ABUNDANCE OF MICROBORING ORGANISMS: ST. JOHN, US VIRGIN ISLANDS JONATHAN ROGERS, University of Wisconsin - Oshkosh Research Advisor: Eric Hiatt MACROBIOEROSION RATES OF IN-SITU CORAL COLONIES: ST. JOHN, U.S. VIRGIN ISLANDS ELIZABETH WHITCHER, Oberlin College Research Advisor: Dennis Hubbard

Keck Geology Consortium Pomona College

185 E. 6th St., Claremont, CA 91711 Keckgeology.org

INTRODUCTION

Over the past four decades, the extent and severity of coral reef decline has increased dramatically. Re-searchers have observed widespread phase shifts within reef ecosystems around the globe, with coral colonies being replaced by vast macroalgal communi-ties (Gardener et al., 2003; Bellwood et al., 2004). In light of climate change and predictions of accelerated sea-level rise, the disappearance of the reef’s primary carbonate producers at alarming rates has become in-creasingly disturbing. Structural complexity of reefs throughout the Caribbean has been greatly reduced since the 1970s (Alvarez-Filip et al., 2011), which has the potential to negatively impact reef organisms that depend on spatial complexity. One of the major ques-tions facing reef scientists today is whether reefs will be able to adapt to accelerating environmental change.

Reef accretion is a delicate balance of constructive processes that contribute carbonate to the reef, destruc-tive agents that break it down, and forces that remove it from the reef. The balance of these elements within a reef can be expressed as a carbonate budget, which takes into account all sources of production and de-struction of carbonate structure on the reef, as well as import and export of carbonate sediments to and from the reef system. Coral growth is the main constructive component of reef building, while activity by bioerod-ing organisms is considered to be the major destructive factor (Tribollet & Golubic, 2011). The term ‘bioero-sion’ was first defined by Neumann (1966), as “the re-moval of lithic substrate by the direct action of organ-isms”. Bioeroders can be separated into epifaunal and infaunal agents. Infaunal borers can be further divided by scale, as microscopic algae, cyanobacteria, and fun-gi, and macroscopic sponges, worms, and molluscs. Individual reefs vary greatly in the relative contribu-tion of each of these major groups to the overall re-moval of coral skeleton (Osorno et al., 2005; Chazottes

MACROBIOEROSION RATES OF IN-SITU CORAL COLONIES: ST. JOHN, U.S. VIRGIN ISLANDS

ElIzabETh WhITChER, Oberlin CollegeResearch Advisor: Dennis Hubbard

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et al., 2002; Pari et al., 1998).

Bioerosion is a taphonomic process that generally be-gins immediately after the death of the coral colony. Studies attempting to quantify rates of bioerosion face the challenge of constraining the duration over which it occurred. While it is relatively simple to quantify the amount of coral skeleton removed by infaunal bioero-sion, it is much more difficult to determine when the coral died, and therefore how long it took to create the end result. Previous studies have dealt with this either by reporting only intensity of boring in natural sub-strates, or by utilizing experimental substrates to con-strain time of exposure. These experimental studies typically use uniform blocks cut from coral skeleton (Osorno et al., 2005; Chazottes et al., 2002; Pari et al., 1998). Their results have been widely accepted as an accurate characterization of bioerosion, but rely on the assumption that these artificial constructs will behave similarly to corals in the reef. This study takes advantage of bimonthly to annual National Park Service monitoring studies to constrain time of death for individual corals. Repeated video monitoring surveys of coral communities have been conducted along a depth gradient in two bays off east-ern St. John in the U.S. Virgin Islands. Video is used to determine time of death and subsequent duration of ex-posure to bioeroders of in-situ colonies of Montastrea annularis. This enables calculation of macro-bioero-sion rates from in-situ coral colonies. The data de-scribed below characterize an important component of natural substrate destruction for these eastern St. John reef communities. In combination with other studies described in this volume, it will be possible to quantify carbonate cycling in St. John reefs and perhaps address questions about the changing balance between carbon-ate production and destruction in the face of accelerat-ing environmental change.

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coral sample, providing an estimate of its age. The time of exposure for the sides of each coral sample was approximated by adding the time before death, based on the age of the sample, to the time since death as determined from the monitoring video. The net rate of bioerosion along the sides of each colony was calcu-lated as the volume removed per unit area on each side divided by the time of exposure (in kg/m2yr-1). To ex-amine spatial variability in bioerosion within colonies, the monitoring video was used to characterize each side of the slab as “cryptic” (i.e., next to another coral, where light, currents and grazing would be restricted) or “exposed” (i.e. facing open water).

RESUlTS

Macrobioeroder communities in Haulover and Lame-shur Bays are overwhelmingly dominated by sponges, with worms and molluscs playing secondary roles in substrate removal (Fig. 2).

METhODS

In July 2011, 74 dead coral colonies were removed with a hammer and chisel along eleven monitoring transects spread over a 4.9 to 18.2 meter depth range at Haulover and Lameshur Bays in St. John (Figs. 1, 2c, in Hubbard and Parsons-Hubbard, this volume). The time of death for each sample was constrained using National Park Service video monitoring data (Fig. 2b in Hubbard, this volume). Four live corals were also collected from depths of 7.6, 10.7, 13.7 and 16.8 me-ters to verify that bioerosion does not occur until death. In the lab, each sample was cut into 3-6 slabs. Both sides of every slab were scanned at high resolution (600-1200 dpi). Of the 75 samples, 33 were selected for which the time of death could be well constrained, usually to early 2006. Only colonies of M. annularis were used. Data were generally collected from the center slab in order to provide a representative assess-ment of bioerosion. Scans of the 33 slabs were imported to CanvasX, where burrows of macroborers were traced and color-coded by phylum (Fig. 1). Borings were identified as sponges, worms, or molluscs, using detailed descrip-tions in Bromley (1978) and Perry & Hepburn (2008). NIH ImageJ was used to determine the total area (cm2) of the borings of each macroboring group, from the top and both sides of each slab. The perimeter of each des-ignated region on the slab (i.e. top, side) was measured with a tape measure, and the area removed by each bio-eroding group was divided by that length to compute area of substrate removed per linear cm of surface (or volume per m2). Volume was converted to mass of carbonate using a density of 1.552 g/cm3 for M. annu-laris. The estimated number of years since death was used to convert total bioerosion into a rate (kg/m2yr-1).

The time of exposure for the tops of the coral colo-nies was reasonably well constrained by the video data. The columnar morphology of M. annularis made determining the duration of exposure of colony sides more complicated, as they were not visible in the video. The rate of linear extension of up to two coral samples from each transect was calculated using annual den-sity banding visible in X-rayed slabs (see Benson, this volume). This rate was applied to the height of each

Figure 1. Burrows of macroborers were traced in Can-vasX and identified as sponges (blue), worms (green), or molluscs (yellow). Images were then imported to NIH ImageJ in order to calculate total area removed by each group.

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Macrobioerosion rates are highly variable throughout the samples that were examined; standard deviations approached computed means in nearly every case. Bioerosion along exposed upper surfaces averaged 0.38 kg/m2yr-1 ± 0.36 (excluding two outliers; Fig. 3). Rates were generally lower in Haulover (0.18 ± 0.13 kg/m2yr-1) than in Lameshur Bay (0.48 ± 0.39 kg/m2yr-1). No depth-related patterns emerged within the data taken from the upper surfaces of colonies (Fig. 3).

The averages for the exposed and cryptic sides of colonies sides were similar to each other (cryptic: 0.19 ±0.15 kg/m2yr-1; exposed: 0.14 ±0.06 kg/m2yr-1: Fig. 4) but lower than the average rate computed for colony tops. However, because the sides encompass larger surface areas that have been exposed to borers for a much longer time, the average total mass of skel-eton removed by bioerosion over the past 15-35 years has been greater along the sides (4.40 ±3.27 g) than on the exposed tops (1.85 ±2.33 g).

Figure 2. Infaunal bioeroder communities in Haulover and Lameshur bays are composed of sponges, worms and molluscs. The average percentage by mass of coral skel-eton removed by each group is indicated, with sponges as the dominant remover of substrate.

Figure 3. Rates of total macrobioerosion (kg/m2yr-1) along the tops of St. John coral samples, plotted against depth (m). Two outliers are not shown here: one sample from 10.7 m with a rate of 3.05 kg/m2yr-1, and one at 18.2 m with a rate of 3.43 kg/m2yr-1.

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transect on St. John were plotted alongside average macrobioerosion rates from eleven studies that used blocks of Porites lobata skeleton, cut to approximately 8x8x5 cm, and deployed on the reef. All but one of the studies (Reaka-Kulda et al., 1996) produced rates similar to or below those of the present study (Fig. 5). Pari et al. (2002) reported rates similar to this study and higher than those of the other shorter-term studies. The bioerosion rates reported for St. John are based on the time of death of individual corals. This study set that point at the last time the coral was seen alive in the monitoring video. However, death was often a gradual process spanning a period of two to six months in late 2005 and early 2006 (as was the case for 26 samples). For seven samples, this interval was as large as two years. The use of ‘last-live’ as the time of death provides a maximum exposure time and, therefore, a minimum value for the rate of bioerosion in the sam-ples. For this reason, differences between estimates of rates provided in this study and those based on shorter-term experimental studies may be even greater than is shown in Figure 5.

These findings are consistent with suggestions that bio-

DISCUSSION

It has long been assumed that shallow-water reefs ac-crete at a faster rate than those in deep water, due to the photosynthetic contribution of zooxanthellae. Howev-er, a recent summary of Holocene reef accretion in the Caribbean reveals that this is not the case (Hubbard, 2009). Data from 151 core intervals collected from fossil reefs that built in paleo-water depths between 0 and 25 meters revealed no statistically significant re-lationship between accretion rates and depth. It was proposed that negative trends in bioerosion and calci-fication with depth might be responsible for this coun-ter-intuitive finding. The St. John samples do not sup-port this hypothesis as it relates to macrobioeroders. Benson (this volume) suggests that deeper water may act as a buffer to adverse temperature and sediment stress, resulting in less substrate being made available for degradation on a regular basis. More research is needed to further explore potential reasons for this lack of an expected depth-related trend in reef accretion.

In order to compare the use of in-situ and experimen-tal substrates, the average rates of bioerosion for each

Figure 4. Rates of total macrobioerosion (kg/m2yr-1) for sides of coral colonies, plotted against depth (m). Sides are cat-egorized as cryptic (immediately adjacent to another coral, where light and currents are reduced) or exposed (facing open water).

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erosion rates increase with duration of exposure (Pari et al., 2002; Kiene & Hutchings, 1994). This temporal pattern may be due to a relationship between maturity of the macroboring community and rate of substrate removal (Kiene & Hutchings, 1994; Hudson, 1977). These results suggest that the balance between con-struction and destruction on reefs will vary temporally following widespread coral mortality, shifting towards net erosion over time as bioerosion rates increase. This pattern is not likely to persist indefinitely, as rates of bioerosion will eventually slow due to limited sub-strate and competition between borers (Rutzler, 1975). Studies examining long-term temporal trends in bio-erosion rates and community evolution are required to confirm these positive trends.

This study shows that bioerosion rates within cryptic spaces (i.e. the sides of coral colonies) of St. John reefs are substantial, highlighting the importance of detailed measurements of reef rugosity in estimating the total

bioerosion occurring on the reef. In reefs built by cor-als with a morphology incorporating a high density of cryptic spaces (such as the columnar form of M. annu-laris), careful measurements of smaller-scale “meso-rugosity” (comparable to “Scale 2” rugosity as defined by Dahl, 1973) will increase calculations of destruc-tion, while measurements based upon quadrats or even chain transects that only consider larger irregularities (“macro-rugosity” or “Scale 1” in Dahl, 1973) on the reef surface will tend to underestimate total bioero-sion. Montastrea has been a major Caribbean frame-builder throughout the Holocene, and has been among the more resistant species during the recent decline. However, the outlook for reefs built by this coral may be grimmer than the view from the surface suggests. The issue of rugosity would be less problematic in reefs containing fewer cryptic spaces, such as those dominated by dome-shaped corals. Therefore, it is im-portant that carbonate budget studies aiming to quan-

Figure 5. Average macrobioerosion rates (kg/m2yr-1) for each depth range at St. John plotted with average macrobioero-sion rates calculated by ten experimental bioerosion studies undertaken around the world. Studies from French Polynesia are shown in orange, the Indian Ocean in blue, Great Barrier Reef in green and this study in red. Reaka-Kulda et al. (1996), a 15-month study from the Galapagos providing an average rate of 2.6 kg/m2yr-1 is not plotted here.

24th Annual Keck Symposium: 2011 Union College, Schenectady, NY

tify total destruction by applying bioerosion rates to calculations of exposed surface area use appropriate measurements of rugosity for the specific reef surface being studied. The importance of morphology in re-lation to bioerosion was recognized by Scoffin et al. (1980), who applied morphology-specific bioerosion rates to substrates when calculating destruction values for their Barbados carbonate budget. Methodologies like these are important in accurately predicting net ac-cretion rates, and assessing the ability of reefs to re-spond to rising sea level.

The intensity of bioerosion within the reef has poten-tially great implications for the resistance of reefs to physical disturbances. Hurricanes have been one of the major destructive forces contributing to loss of struc-tural complexity on Caribbean reefs over the past four decades (Alvarez-Filip et al., 2011). The present study shows that due to their large surface area and long time of exposure, the sides of M. annularis colonies are the location of major skeletal loss within the colony. This likely weakens many colonies at their base, and may leave large portions of the reef highly vulnerable to the physical damage inflicted by hurricanes. Due to the ecological impacts of diminishing structural complex-ity in Caribbean reefs, these results are highly relevant to predicting the future of reefs and the myriad species that depend on their spatial complexity.

CONClUSIONS

This study provides an estimation of macrobioerosion rates from natural, in-situ substrates. The results sup-port the following conclusions:

1)Rates of bioerosion in top surfaces of coral colonies ranged between 0.006 kg/m2yr-1 and 3.43 kg/m2yr-1, with an average of 0.38 ± 0.36 kg/m2yr-1. In the sides of colonies, bioerosion ranged from 0.02 kg/m2yr-1 to 0.57 kg/m2yr-1, with an average of 0.17 ± 0.14 kg/m2yr-1.

2) Macrobioerosion rates in St John coral communi-ties are not related to depth. If St. John is typical of Caribbean reefs, a different explanation must be found for the lack of a relationship between accretion and depth during the Holocene.

3) Average macrobioerosion rates calculated from in-situ coral substrates in this study exceed average rates in the majority of short-term experimental studies, supporting previous suggestions that bioerosion rates increase with duration of exposure.

4) Macrobioerosion in reefs dominated by M. annu-laris is a three dimensional process, with a significant amount of boring occurring along exposed and cryptic sides of the colonies, in addition to the exposed upper surface of the reef. While rates of macrobioerosion per unit area are higher on the reef surface, the net mass of material removed from within the reef is greater ow-ing to a longer time of exposure and greater surface area. This has important implications for the structural strength of reefs, and reduced spatial complexity due to physical destruction by hurricanes.

5) These findings highlight the importance of using detailed measurements of reef rugosity in determining the surface area exposed to bioerosion. This is espe-cially important in reefs dominated by corals that cre-ate a high quantity of cryptic spaces, such as M. An-nulariss in the Caribbean. Carbonate budgets that take this into account will recognize a far greater amount of destruction than those using large scale measurements of rugosity.

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