Sr/Ca, U/Ca, and stable isotope data and bimonthly records of Ogasawara coral core OGA-02-1

Instrumental climate observations provide robust records of global land and ocean temperatures during the twentieth century. Unlike for temperature, continuous salinity observations in the surface ocean are scarce prior to 1970, and the magnitude of salinity changes during the twentieth century is largely unknown. Surface ocean salinity is a major component in climate dynamics, as it influences ocean circulation and water mass formation. Here we present an annually resolved reconstruction of salinity variations in the surface waters of the western subtropical North Pacific Ocean since 1873, based on bimonthly records of d18O, Sr/Ca, and U/Ca in a coral from the Ogasawara Islands. The reconstruction indicates that an abrupt regime shift toward fresher surface ocean conditions occurred between 1905 and 1910. Observational atmospheric data suggest that the abrupt freshening was associated with a weakening of the winds that drive the Kuroshio Current system and the associated subtropical gyre circulation. We note that the abrupt early-twentieth-century freshening in the western subtropical North Pacific precedes abrupt climate change in the northern North Atlantic by a few years. The potential for abrupt regime shifts in surface ocean salinity should be considered in climate predictions for the coming decades.

Chronology and stable carbon isotopic ratios of sediments from the Western Pacific Warm Pool region

Instrumental data suggest that major shifts in tropical Pacific atmospheric dynamics and hydrology have occurred within the past century, potentially in response to anthropogenic warming. To better understand these trends, we use the hydrogen isotopic ratios of terrestrial higher plant leaf waxes (DDwax) in marine sediments from southwest Sulawesi, Indonesia, to compile a detailed reconstruction of central Indo-Pacific Warm Pool (IPWP) hydrologic variability spanning most of the last two millennia. Our paleodata are highly correlated with a monsoon reconstruction from Southeast Asia, indicating that intervals of strong East Asian summer monsoon (EASM) activity are associated with a weaker Indonesian monsoon (IM). Furthermore, the centennial-scale oscillations in our data follow known changes in Northern Hemisphere climate (e.g., the Little Ice Age and Medieval Warm Period) implying a dynamic link between Northern Hemisphere temperatures and IPWP hydrology. The inverse relationship between the EASM and IM suggests that migrations of the Intertropical Convergence Zone and associated changes in monsoon strength caused synoptic hydrologic shifts in the IPWP throughout most of the past two millennia.

Paleoceanography of sediment cores SO202-27-6 and MD01-2416

The glacial-to-Holocene evolution of subarctic Pacific surface water stratification and silicic acid (Si) dynamics is investigated based on new combined diatom oxygen (d18Odiat) and silicon (d30Sidiat) isotope records, along with new biogenic opal, subsurface foraminiferal d18O, alkenone-based sea surface temperature, sea ice, diatom, and core logging data from the NE Pacific. Our results suggest that d18Odiat values are primarily influenced by changes in freshwater discharge from the Cordilleran Ice Sheet (CIS), while corresponding d30Sidiat are primarily influenced by changes in Si supply to surface waters. Our data indicate enhanced glacial to mid Heinrich Stadial 1 (HS1) NE Pacific surface water stratification, generally limiting the Si supply to surface waters. However, we suggest that an increase in Si supply during early HS1, when surface waters were still stratified, is linked to increased North Pacific Intermediate Water formation. The coincidence between fresh surface waters during HS1 and enhanced ice-rafted debris sedimentation in the North Atlantic indicates a close link between CIS and Laurentide Ice Sheet dynamics and a dominant atmospheric control on CIS deglaciation. The Bølling/Allerød (B/A) is characterized by destratification in the subarctic Pacific and an increased supply of saline, Si-rich waters to surface waters. This change toward increased convection occurred prior to the Bølling warming and is likely triggered by a switch to sea ice-free conditions during late HS1. Our results furthermore indicate a decreased efficiency of the biological pump during late HS1 and the B/A (possibly also the Younger Dryas), suggesting that the subarctic Pacific has then been a source region of atmospheric CO2.

Effects of ocean acidification on population dynamics and community structure of crustose coralline algae

Calcification and growth of crustose coralline algae (CCA) are affected by elevated seawater pCO2 and associated changes in carbonate chemistry. However, the effects of ocean acidification (OA) on population and community-level responses of CCA have barely been investigated. We explored changes in community structure and population dynamics (size structure and reproduction) of CCA in response to OA. Recruited from an experimental flow-through system, CCA settled onto the walls of plastic aquaria and developed under exposure to one of three pCO2 treatments (control [present day, 389±6 ppm CO2], medium [753±11 ppm], and high [1267±19 ppm]). Elevated pCO2 reduced total CCA abundance and affected community structure, in particular the density of the dominant species Pneophyllum sp. and Porolithon onkodes. Meanwhile, the relative abundance of P. onkodes declined from 24% under control CO2 to 8.3% in high CO2 (65% change), while the relative abundance of Pneophyllum sp. remained constant. Population size structure of P. onkodes differed significantly across treatments, with fewer larger individuals under high CO2. In contrast, the population size structure and number of reproductive structures (conceptacles) per crust of Pneophyllum sp. was similar across treatments. The difference in the magnitude of the response of species abundance and population size structure between species may have the potential to induce species composition changes in the future. These results demonstrate that the impacts of OA on key coral reef builders go beyond declines in calcification and growth, and suggest important changes to aspects of population dynamics and community ecology.