Southern High Latitude Climate and Internal Variability Influence on Eastern Equatorial Pacific Thermostad Temperatures during the Holocene

Abstract:

Subantarctic Mode Water (SAMW) plays a critical role in transporting heat, nutrients and CO₂ from the Southern Ocean to the base of the equatorial thermocline in the eastern equatorial Pacific (EEP). In turn, the heat and nutrients storage below the thermocline (~100-500 m) in the EEP can exert large effect on Earth’s climate through ocean-atmosphere heat and CO₂ exchange. Here we present a centennially resolved Holocene subsurface temperature reconstruction using Mg/Ca of Neogloboquadrina dutertrei from the eastern equatorial Pacific (EEP). In the EEP N. dutertrei calcifies at about ~125 m which is below the Equatorial Undercurrent and the upper limit of the thermostad water in this region. During the early Holocene N. dutertrei temperatures vary between 14.5˚C and 16.5˚C, whereas by 8 kyr B.P. the temperature drops to 13˚C. The cooling by 8 kyr is also observed in the bottom water (~400 m) temperature reconstructions from Uvigerina spp. The early Holocene cooling of the thermostad water is likely linked to southern high latitude climate variability attributed to changes in the southern westerly winds (SWW). We posit that a more southern position of the SWW in the early Holocene increased the influence of warm subtropical water into the formation region of and thereby warming SAMW. Additionally, a southward position of the SWW increased SAMW production causing the southern high latitude signal to reach farther into the EEP. After 8 ky, thermostad temperatures show millennial and centennial variability, with low temperature between 4.8-3.6 kyr followed by high temperatures during the next 500 years. High resolution record of the last 2,000 years also show multidecadal to multicentennial thermostad temperature variability. The timing of this variability does not follow the Northern Hemisphere temperature variability including the Little Ice Age and Medieval Climate Anomaly. We conclude that in absence of a strong climate signal from the southern high latitudes during the mid to late Holocene the centennial variability in the EEP hydrography is dominated by intrinsic variability.