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Pliocene Stratigraphy and the Impact of Panama Uplift on Changes in Caribbean and Tropical East Pacific Upper Ocean Stratification (6 – 2.5 Ma)

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Pliocene Stratigraphy and the Impact of Panama Uplift on Changes in Caribbean and Tropical East Pacific Upper Ocean Stratification (6 – 2.5 Ma) PLIOCENE STRATIGRAPHY AND THE IMPACT OF PANAMA UPLIFT ON CHANGES IN CARIBBEAN AND TROPICAL EAST PACIFIC UPPER OCEAN STRATIFICATION (6 – 2.5 MA) PLIOZÄNE STRATIGRAPHIE UND DER EINFLUSS DES PANAMA-SEEWEGES AUF DIE STRATIFIZIERUNG DER OBERFLÄCHEN-WASSERMASSEN IN DER KARIBIK UND IM TROPISCHEN OST-PAZIFIK (6 – 2.5 MA) Kumulative Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Silke Steph Kiel, 2005 PLIOCENE STRATIGRAPHY AND THE IMPACT OF PANAMA UPLIFT ON CHANGES IN CARIBBEAN AND TROPICAL EAST PACIFIC UPPER OCEAN STRATIFICATION (6 – 2.5 MA) PLIOZÄNE STRATIGRAPHIE UND DER EINFLUSS DES PANAMA-SEEWEGES AUF DIE STRATIFIZIERUNG DER OBERFLÄCHEN-WASSERMASSEN IN DER KARIBIK UND IM TROPISCHEN OST-PAZIFIK (6 – 2.5 MA) Kumulative Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Silke Steph Kiel, 2005 158 pages 52 figures 10 tables 8 appendices Referent: ________________________________________ Koreferent: ________________________________________ Tag der Disputation: ________________________________________ Zum Druck genehmigt, Kiel, den: ________________________________________ Der Dekan: ________________________________________ I ABSTRACT This thesis examines the closure history of the Central American Seaway (CAS) and its effect on changes in ocean circulation and climate during the time interval from ~6 – 2.5 Ma. It was accomplished within the DFG Research Unit "Impact of Gateways on Ocean Circulation, Climate and Evolution" at the University of Kiel. Proxy records from Ocean Drilling Program (ODP) Sites 999 and 1000 (Caribbean), and from ODP Sites 1237, 1239 and 1241 (low-latitude east Pacific) are developed and examined. In addition, previously established proxy data from Atlantic Sites 925/926 (Ceara Rise) and 1006 (western Great Bahama Bank) and from two east Pacific sites (851, 1236) are included for interpretations. The main objectives of this study are (1) to acquire a consistent stratigraphic framework for all sites, (2) to reconstruct Pliocene changes in Caribbean and tropical east Pacific upper ocean water masses (i.e. temperature, salinity, thermocline depth), and (3) to identify potential underlying forcing mechanisms. The stratigraphic framework was established by tuning benthic stable isotope records to orbitally tuned reference records (Sites 1241, 925/926). The orbital tuning procedure (Sites 1241, 1237) is based on matching cyclic variations in proxy records like benthic stable isotopes, percentage sand of the carbonate fraction, and shipboard logging data to the orbital solution of Laskar et al. (1993). The tuning results add up to one consistent benthic 18O stratigraphy for all Pliocene Caribbean and tropical east Pacific sediment sections. Accordingly, they provide an excellent opportunity for comparisons of Pliocene proxy records from different oceanic areas. The assessment of Pliocene changes in Caribbean and tropical east Pacific upper ocean water mass signatures is achieved by a comparison of 18O records from shallow- and deep-dwelling planktonic foraminifers, indicative of salinity and temperature changes. These 18O records are partly combined with Mg/Ca-derived temperature records from the same foraminiferal species. Pronounced sea surface salinity (SSS) gradients between the two Caribbean sites (999, 1000) after 4.7 – 4.2 Ma with lower salinities closer to the final gateway region indicate that the lateral expansion of the Pacific low salinity tongue into the Caribbean was strongly reduced. Besides, the inner-Caribbean SSS gradients reveal a strong variability on precessional time scales after 4.4 Ma, with higher SSS gradients during northern hemisphere summer insolation maxima. This suggests that the volume transport through the CAS varied on precession cycles, implying higher contributions of low-salinity Pacific inflow during northern hemisphere summer insolation minima, and a stronger influence of high-salinity Atlantic water masses during northern hemisphere summer insolation maxima. 18 With OG. sacculifer values being significantly lower in the equatorial western Atlantic (Site 925, Ceara Rise) than at the Caribbean sites after 4.4 Ma, the possibility of a transfer of high-salinity water masses from 18 the South Atlantic to the Caribbean can be excluded. Similar OG. sacculifer values in the central Caribbean (Site 1000) and the Caribbean outflow region (Site 1006) indicate that high-salinity water masses left the Caribbean via the Florida Current, thereafter feeding the Gulf Stream. The combination of both results suggests the North Atlantic subtropical gyre as a possible source for Caribbean high-salinity surface waters during the Pliocene. A warming and/or deepening of the Caribbean thermocline after 4.5 Ma indicates a strengthening of the Caribbean/West Atlantic Warm Pool. Concomitant to the thermocline-deepening in the Caribbean, the tropical east Pacific thermocline shoaled with a major subsurface cooling step between 4.5 and 4.0 Ma (Site 1241). Temperatures within the thermocline, though, remained relatively stable throughout the Pliocene, as indicated by Mg/Ca temperature reconstructions from Site 1241. The observed changes are in good agreement with numerical modeling results adressing the closure of the CAS that were carried out within the framework of the Research Unit "Ocean Gateways". Strong mixed-layer temperature and/or salinity (18O) and nutrient (13C) gradients between the equatorial east Pacific (Site 1239) and the east Pacific North Equatorial Counter Current region (Site 1241), especially after 4.5 Ma, do not support the scenario of a "permanent El Niño-like state" during the Pliocene. II KURZFASSUNG Die Auswirkungen der Schließung des Panama-Seeweges auf Ozeanzirkulation und Klima für den Zeitabschnitt von ~6 – 2.5 Mio. J.v.h. (Millionen Jahren vor heute) stehen im Mittelpunkt dieser Studie, die innerhalb der DFG Forschergruppe "Ozeanpassagen" an der Universität Kiel durchgeführt wurde. Im Rahmen dieser Arbeit wurden Proxy-Kurven von Ocean Drilling Program (ODP) Sites 999 und 1000 (Karibik) und von ODP Sites 1237, 1239 und 1241 (tropischer Ost-Pazifik) erstellt und untersucht. Bereits vorhandene Proxy-Daten vin den atlantischen ODP Sites 925/926 (Ceara Rise) und 1006 (Bahama-Bank) sowie von zwei ost-pazifischen Sites (851, 1236) wurden für Vergleiche und Interpretationen herangezogen. Die wesentlichen Zielsetzungen umfassten: (1) die Erarbeitung einer einheitlichen Stratigraphie, (2) die Rekonstruktion von pliozänen Veränderungen in den oberflächennahen Wassermassen der Karibik und des tropischen Ost-Pazifiks (z.B. Temperatur, Salzgehalt, Thermoklinentiefe) und (3) die Abschätzung der Steuerungsmechanismen für diese Veränderungen. Das stratigraphische Gerüst wurde durch die Anpassung benthischer Sauerstoff-Isotopenkurven an astronomisch kalibrierte Referenzkurven (Sites 1241, 925/926) erstellt. Die astronomische Kalibrierung (Sites 1237, 1241) basiert auf der Anpassung von Proxy-Daten (z.B. benthischen 13C Kurven, Sandgehalten (%) der Karbonatfraktion und Kernlogging-Daten) an berechnete Orbitalparameter. Die daraus resultierende einheitliche Stratigraphie ergab die Möglichkeit, die pliozänen Sedimentkerne der Karibik und des Pazifiks direkt miteinander zu vergleichen. Die Rekonstruktion pliozäner Veränderungen in den oberflächennahen Wassermassen basiert auf einem Vergleich von 18O-Kurven für flach- und tieflebende planktonische Foraminiferen (Anzeiger für Temperatur und Salzgehalt). Diese wurden zum Teil mit Mg/Ca-Temperatur-Rekonstruktionen für die gleichen Foraminiferenarten kombiniert. Nach 4.7 – 4.2 Mio. J.v.h. entwickelten sich ausgeprägte Unterschiede im Salzgehalt der oberflächennahen Wassermassen zwischen den karibischen Sites 999 und 1000. Höhere Salzgehalte in größerer Entfernung zu den letzten Öffnungen des Panama-Seeweges weisen darauf hin, dass die laterale Ausdehnung der salzärmeren pazifischen Wassermassen in die Karibik hinein zu dieser Zeit stark eingeschränkt war. Außerdem zeigen die inner-karibischen Unterschiede im Salzgehalt der oberflächennahen Wassermassen nach 4.4 Mio. J.v.h. eine ausgeprägte Variabilität auf Präzessions-Zyklen. Dabei gehen höhere Unterschiede im Salzgehalt zwischen Sites 999 und 1000 in der Regel mit Maxima in der Nordhemisphären- Sommerinsolation einher. Dies deutet auf einen stärkeren Einstrom von salzärmeren pazifischen Oberflächen-Wassermassen während Minima – und auf einen verstärkten Einfluss von salzreicheren atlantischen Wassermassen während Maxima in der Nordhemisphären-Sommerinsolation hin. Ein Transfer von salzreicheren Wassermassen aus dem Südatlantik in die Karibik nach etwa 4.4 Mio. J.v.h. kann ausgeschlossen werden, da die 18O-Werte von G. sacculifer im äquatorialen West-Atlantik (Ceara Rise, Site 925) deutlich geringer waren als in der Karibik (Site 1000). Ähnliche 18O-Werte von G. sacculifer in der zentralen Karibik (Site 1000) und in der karibischen Ausstrom-Region (Site 1006) zeigen jedoch, dass salzreiche Oberflächen-Wassermassen aus der Karibik den Golfstrom speisten. Eine Verknüpfung dieser Ergebnisse schlägt die nordatlantische subtropische Gyre als Quellregion für salzreiche karibische Oberflächen-Wassermassen vor. Eine Erwärmung der karibischen Thermokline nach 4.5 Mio. J.v.h. deutet auf eine Verstärkung des atlantisch-karibischen Warmwasser-Pools hin. Gleichzeitig zeigt eine starke Abkühlung am Fuß der photischen
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