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Validating Annual Growth Bands of Deep Sea Black Corals from the Gulf of Mexico and Southeastern United States
Leslye Mohon ([email protected])1, E. Brendan Roark1, Renald Guillemetter2, Nancy Prouty3, Steve Ross4
Deep-‐sea black corals have the poten2al to be u2lized as a proxy record of historical and biogeochemical changes in worlds oceans (Williams et al. 2006). These black corals (Leiopathes sp.) grow in a tree like fashion by deposi2ng poten2al annual growth bands. By valida2ng that growth bands are in fact annual, the ages and growth rates of black corals from the Gulf of Mexico and Southeastern United Stated (SEUS) can be determined. Iodine concentra2on in black corals also have the poten2al to be used as a new da2ng method to determine the lifespan and growth rates of black corals. When the iodine chronology is combined with radiocarbon measurements, con2nuous records of ocean ven2la2on changes are possible.
Introduc)on
Visual Counts • Scanning electron microscope (SEM) was used to develop images (900x)
that shows growth bands. Iodine and BSE Counts • The number of backscaPer electrons (BSE) reaching the detector is
propor2onal to the mean atomic number of the sample. • Iodine and BSE were measured along radial transects using the SEM at 1
µm spot intervals.
Figure 2. Prouty et al., (2011) analyzed black corals (GOM-‐JSL04-‐4734-‐BC1) by o b s e r v i n g s c a n n i n g e l e c t r o n microscope (SEM) 40x images and counted an average age of 576 bands c ompa r e d t o t h e c a l c u l a t e d radiocarbon derived life span of 670 ±40 yrs.
The USGS Terrestrial, Marine, and Freshwater Environments-‐Outer Con2nental Shelf Ecosystem Program and USGS Coastal and Marine Geology Program and a grant to EBR from The Norman Hackerman Advanced Research Program supported this work.
Acknowledgments
§ Confident that the growth bands in black corals are indeed annual. § Iodine and visual counts along with BSE are suitable methods to
calculate growth rates and life spans of black corals § Ongoing research will replicate and validate reservoir ages to account
for the variability in ocean circula2on.
Conclusions
Figure 4. This is an SEM image of a black coral with iodine and BSE counts overlaid on top. This shows that Iodine along with BSE match the visual growth bands. When conduc2ng visual counts, different observers will have different defini2on of what is a growth band so iodine may serve as a less subjec2ve da2ng method.
Figure 6. The top panel represents an en2re radial transect of iodine and BSE data. The boPom panel represents clearly defined iodine and BSE peaks ader a threshold of 1000 was implemented over a 1 mm distance. This was to remove the base line noise associated with the darker parts of the SEM image in order to more easily iden2fy the iodine and BSE peaks.
Study Site and Samples
Figure 7. Iodine can be u2lized as an independent chronology which allows for the calcula2on of radiocarbon reservoir ages. This con2nuous high resolu2on 600 year record of reservoir ages from Viosca Knoll in the Gulf of Mexico shows a high degree of variability. Reservoir ages calculated from modern (Wagner et al., 2009) and fossil mid-‐Holocene (Druffel et al., 2008) tropical (surface) corals are included for comparison.
Possible explana2ons for the variability in reservoir ages: • Strong and weak Yucatan current, which turns on and off the regional
upwelling associated with gyre forma2on in the Gulf of Mexico (Fig. 8). • Proximity to the Mississippi river and associated changes in discharge
could influence reservoir ages. • Falling atmospheric ∆14C values can result in transient lower reservoir
ages (Druffel et al. 2008).
• 250 µm by 250 µm • Magnified by 900x • SEM and PIXIE analyses
Figure 3. Nowak et al., (2009) examined the concentra2on of different elements in black corals. They found that higher iodine concentra2ons appear to be closely associated with growth bands iden2fied in SEM images. A SEM and micro-‐Par2cle induced x-‐ray Emission ( u -‐ P I X I E ) w a s u s e d a t 9 0 0 x magnifica2on.
Figure 1. Map of two loca2ons where the black corals were collected. In the Gulf of Mexico the corals came from the Viosca Knoll or the Desoto canyon. There are three banks that make up the Southeastern United States (SEUS): Savannah Bank, Stetson Bank, and Jacksonville Bank.
Valida)ng Annual Growth Bands
Data & Methods
Figure 5. The comparison of four methods of obtaining the life span and growth rates of black corals is presented. These results show that the ages and growth rate es)mates using iodine peaks closely match the radiocarbon results and validates that the these bands and peaks in iodine are indeed annual chronometers.
GOM-JSL04-4734-BC1
1 Department of Geography, Texas A&M University, College Sta2on, TX 77840 2Department of Geology and Geophysics, Electron Microbe Lab, Texas A&M University, College Sta2on, TX 77840
3US Geological Survey, 400 Natural Bridges Drive Santa Cruz, CA 95060 4Center for Marine Science, University of North Carolina at Wilmington, Wilmington, NC 28409
Figure 8. When the Yucatan current is strong, it invades the Gulf of Mexico forming a loop current and mesoscale gyres. This process0 increases upwelling which in turn increases reservoir ages. When the Yucatan current is weak it turns sharply into the Florida Straights and no gyres or upwelling occurs in the Gulf of Mexico decreasing the reservoir ages recorded in the black corals (Druffel et al. 2008).
Druffel, E. R. M., L. F. Robinson, S. Griffin, R. B. Halley, J. R. Southon & J. F. Adkins (2008) Low reservoir ages for the surface ocean from mid-‐Holocene Florida corals. Paleoceanography, 23, PA2209. Nowak, D., M. Florek, W. Kwiatek, J. Lekki, P. Chevallier, A. Hacura, R. Wrzalik, B. Ben-‐Nissan, R. Van Grieken, A. Kuczumow (2009). Morphology and the Chemical Make-‐Up of the Inorganic Components of Black Corals. Materials Science and Engineering, 29, 1029-‐1038. Williams, B., M. J. Risk, S. W. Ross & K. J. Sulak (2006) Deep-‐water an2patharians: Proxies of environmental change. Geology, 34, 773-‐776
References
Measured 14C age – IntCal09 Atm 14C age (yrs. BP –Iodine age model) = 14C reservoir age
Background
Valida)ng Annual Growth Bands