Eastern  equatorial  Pacific  primary  productivity  during  the  Pliocene-­‐Pleistocene  climate  transition

In  chapter  8  we  reconstructed  the  evolution  of  the  primary  productivity,  biogeochemical  cycles   and   oceanic   conditions   from   ODP   sites   1239   and   1240,   both   situated   in   the   equatorial  

Similar  productivity  records  have  been  reported  elsewhere  in  the  central  and  Eastern  tropical   Pacific.   For   instance  Farrell   et   al.  (1995)   reported   elevated   opal   percentages   and   AR   in   CHAPTER  9  

Galapagos   region   between   3   and   1.5   Ma,   with   a   distinct   maximum   at   about   1.9   Ma.   This   maximum,  as  well  as  in  ODP  Sites  1239  and  1240,  corresponds  to  a  minimum  in  %CaCO3  and   CaCO3   flux   (Figure   9.3).   Since   the   export   of   carbonate   from   the   surface   ocean   is   a   more   efficient  transporter  of  organic  carbon  to  the  deep  sea  than  diatom  silica  because  of  its  greater   abundance  and  density  (e.g.  Armstrong  et  al.,  2002;  Francois  et  al.,  2002),  carbonate  contents   decrease   might   indicate   either   (1)   lower   calcareous   plankton   production   or   (2)   lack   of   preservation.  In  the  EEP,  Murray  et  al.    (1995)  examined  preservation  data  to  relative  influence   of  dissolution  on  the  alteration  of  the  record  of  carbonate  production.  They  concluded  that  the   minimum   in   CaCO3   AR   is   related   to   production   rather   than   dissolution   hypothesizing   that   during  those  intervals  of  opal  accumulation  maxima  at  times  of  low  CaCO3  AR  (I.e.  1.5-­‐2.4  Ma)   implies   that   siliceous   organisms   dominated   production,   especially   because   only   a   small   proportion  (approx.  10%)  of  the  opal  rain  is  presently  preserved  at  those  sites.  

Figure  9.  3  Sedimentary  records  of  ODP  Sites  846  and  850,  from  Farrell  et  al.  (1995).  (f)  Opal  AR  records  from  the    

Galapagos   region   (Sites   846)   and   the   equatorial   zone   (Site   850)   show   the   shift   in   the   locus   of   maximum   opal   accumulation  at  4.4  Ma,  associated  to  the  final  closure  of  the  Panamanian  seaway.  Opal  AR  declined  sharply  after   4.4   Ma   in   the   Pacific   Basin   and   increased   in   the  Galapagos   region.   Also   shown   is   a   common   increase   near   2   Ma   associated  to  a  (b)  decrease  in  %CaCO3  observed  at  several  EEP  sites  .  

Being  coccolithophores  one  of  the  most  important  pelagic  calcifying  organisms  in  the  present   ocean,   the   high   abundance   of  C37   alkenones   between,   1.5   and   2.4   Ma  (Figure   8.2b),   results   contrasting   with  the   EEP   minima   in   %CaCO3   and   CaCO3   flux   (Figure   9.3).   However,   The   abundance  of  alkenones  in  sediments  is  a  function  of  both  the  amount  of  haptophyte  export   productivity   from   the   overlying   water   column   and   organic   matter   preservation,   itself   dependent  on  sediment  redox  conditions  and  accumulation  rates.  Concerning  the  haptophyte   productivity,  Bolton  et  al.  (2010)  found  that  for  the  EEP  the  Pliocene  nannofossil  assemblage   patterns  primarily  reflect  the  nutrient  and  temperature  affinities  of  the  grouped  species  rather   than   a   preservational   signal   and   that   the   large-­‐amplitude   of   Glacial–Interglacial   changes   in   %CaCO3   during   the   Pliocene   partially   result   from   productivity   variations,   i.e.,   the   massive  

input  of  biogenic  silica  during  glacials  dilutes  the  %CaCO3  content  of  the  sediment  record.  

In  summary,  the  significant  correspondence  between  a  regional  EEP  long-­‐term  high  biogenic   opal   content   (E.g.   Farrell   et   al.,   1995)   associated   to   a   long-­‐term   increase   in  alkenone-­‐

synthesizing   coccolithophores  (E.g.  Lawrence   et   al.,   2006;   Dekens   et   al.,   2007)  and   TOC   between  2.4  and  1.5  Ma  provides  robust  evidence  that  the  Plio-­‐Pleistocene  transition  pattern   are  not  significantly  skewed  by  dissolution  but  are  rather  a  clear  signal  of  an  enhanced  ratio  of   siliceous  to  coccolithophore  productivity  resulting  from  a  common  forcing.    

  Pliocene/early  Pleistocene  cooling,  neither  the  weak  biological  production  during  the  cooling   interval   or   since   the   mid-­‐Pleistocene.   Moreover,   it   has   been   recently   shown   that   the   wind-­‐

driven   upwelling   activity   in   the   Pacific   likely   strengthened   since   2   Ma   ago   due   to   the   intensification  of  the  tropical/subtropical  atmospheric  circulation  (Etourneau  et  al.,  2010).  In   ODP  Sites  1239  and  1240  records,  the  intensification  of  the  upwelling  coincided  with  a  decline  

latitude  upwelling  systems  resulting  in  the  proliferation  of  fast-­‐growing  diatoms.  Given  that  Fe   is   the   major   limiting   factor   in   the   modern   EEP,   we   assert   that   Fe   was   a   limiting   factor   for   biological  production  all  over  the  Plio-­‐Pleistocene  climate  transition.  The  comparison  between   TN,  TOC  and  Fe  contents  at  Site  1239  revealed  striking  similarities  over  the  last  3  Ma  (Figure  

corresponded   to   reduced   Fe   supply   due   to   the   intensification   of   the   Walker   circulation,   a   progressive  switch  to  drier  conditions  over  the  EP.  However,  our  results  do  not  support  this   simple  scheme.  Since  3  Ma,  dry  conditions,  basically  encountered  during  normal  or  La  Niña-­‐like   conditions,  mostly  dominated  the  EP.  Solely  the  peak  between  2.4  and  1.6  Ma  suggests  more   humid  conditions  associated  to  a  higher  precipitation  rate.  

9.2.  Outlook  and  future  perspectives    

The  manuscripts  presented  as  part  of  this  thesis  used  a  cluster  of  paleoceanographic  proxies  in   order  to  answer  some  specific  scientific  questions  for  which  the  project  “The  missing  link  to   understand   Plio-­‐Pleistocene   changes   in   southeast   Pacific   oceanography,   productivity,   and   El   Niño  behavior  –  SE  trade  wind  strength  and  its  dust  transport”  was  undertaken.  While  those   questions  were  to  a  great  extent  answered,  other  issues  of  topical  interest  emerged.  Mainly  as   a   consequence   of   the   findings   reported   here.   In   this   context,   some   future   perspectives   that   may  address  the  newly  emerged  questions  are  summarized  below.  

9.2.1.  Oxygen  isotopes  of  planktonic  foraminifera  

The   general   pattern   of   the   differences   between   “equilibrium   calcite”   and   fossil   foraminifera   δ¹⁸O   demonstrates   that   the   relationship   between   them   is   complex   and   depends   on   local   hydrography.   From   our   study,   it   seems   unlikely   that   a   transfer   function   in   form   of   a   simple   linear   regression   equation   is   the   appropriate   tool   to   correct   for   these   distortions   in   paleoceanographic  reconstructions  of  complex  areas  such  as  the  eastern  tropical  Pacific.  We   realized  that  the  current  database  of  δ¹⁸O  of  seawater  for  the  eastern  Pacific  still  need  to  be   improved  significantly  with  measurements  in  the  easternmost  area  and  across  the  equatorial   front.    

Although  several  δ¹⁸O  gradients  between  species  indeed  show  changes  indicative  of  variations   of   ocean   stratification   across   the   equatorial   front,   this   approach   can   be   complicated   by   the   fact   that   foraminifera   are   not   restricted   to   a   certain   depth   level,   but   can   also   change   their   habitat  depth.  For  example,  Sautter  and  Thunell  (1991)  have  shown  that  both  N.  dutertrei  and   N.  pachyderma  follow  isothermal  ranges,  migrating  to  shallower  waters  during  upwelling  and   subsequently  descending  after  upwelling.  Additionally,  the  use  of  sediment  samples  might  be   biased  since  (1)  not  all  the  species  of  foraminifera  have  the  same  seasonality;  (2)  foraminiferal   tests   are   exposed   to   post-­‐depositional   mixing   (I.e.   bioturbation,   winnowing   and   lateral   migration  along  the  ocean  floor);  and  (3)  some  can  experience  calcite  dissolution.  Therefore,   further   research   is   clearly   needed   concerning   the   isotopic   composition   in   living   planktonic   foraminifera   in   order   to   depict   a   clearer   relationship   between   the   “equilibrium   calcite”   and   foraminiferal   δ¹⁸O,   avoiding   artifacts   caused   by   seasonality,   sediment   mixing   and   calcite   enriching  in  ¹⁸O  during  ontogenesis  or  gametogenesis.  Furthermore,  it  is  important  to  develop   much  more  research  on  the  ecology  and  isotopic  composition  of  living  planktonic  foraminifera   and  its  distribution  across  the  Equatorial  front  during  different  seasons.  

9.2.2.  Glacial-­‐interglacial  terrigenous  delivery  and  continental  hydrological  balance.  

The  specific  mechanism  behind  the  disproportion  in  the  magnitude  of  lithogenic  supply  among   interglacials   to   the   EEP   (ODP   Site   1239)   is   unknown.   Differences   in   atmospheric   CO2   concentrations,  astronomical  forcing  and  glacial  ice-­‐volume  (see  Tzedakis  et  al.,  2009)  plausibly   fostered  variations  in  moisture  advection  to  the  ITCZ  and  to  the  continent  hence  stimulating   variations  in  precipitation,  vegetation  cover  and  fluvial  suspended  loads  that  are  reflected  in   the  variable  magnitude  between  the  different  interglacials  of  terrestrial  input  to  the  EEP.  To