Validating Annual Growth Bands of Deep Sea Black Corals from the Gulf of Mexico and Southeastern United States

Leslye Mohon (lmohon@neo.tamu.edu)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