Planktonic foraminifera Mg/Ca paleotemperatures and oxygen isotopes of ODP Hole 175-1075C

High resolution planktonic foraminifera Mg/Ca paleotemperatures and oxygen isotopes of seawater of Ocean Drilling Program (ODP) Site 1078 (off Angola) have been reconstructed and reveal insights into the seasonal thermal evolution of the Angola Current (AC), the Angola-Benguela Front (ABF), and the Benguela Current (BC) during the last glacial (50-23.5 ka BP). Special emphasis is put on time intervals possibly associated with the North Atlantic Heinrich Stadials (HS), which are thought to lead to an accumulation of heat in the South Atlantic due to a reduction of the Atlantic Meridional Overturning Circulation (AMOC). Within dating uncertainties, Globigerinoides ruber (pink) Mg/Ca-based sea surface temperature (SST) estimates that represent southern hemisphere summer surface conditions show several warming episodes that coincide with North Atlantic HS, thus supporting the concept of the bipolar thermal seesaw. In contrast, the Mg/Ca-based temperatures of Globigerina bulloides, representing the SST of the ABF/BC system during southern hemisphere winter, show no obvious response to the North Atlantic HS in the study area. We suggest that surface water cooling during the winter season is due to enhanced upwelling or upwelling of colder water masses which has most likely mitigated a warming of the ABF/BC system during HS. We further speculate that the seasonal asymmetry in our SST record results from seasonal differences in the dominance of atmospheric and oceanic teleconnections during periods of northern high latitude cooling.

Monthly Bonaire coral Sr/Ca and oxygen isotope data from 118 kyr ago (coral BON-5-D)

The end of the last interglacial period, ~118 kyr ago, was characterized by substantial ocean circulation and climate perturbations resulting from instabilities of polar ice sheets. These perturbations are crucial for a better understanding of future climate change. The seasonal temperature changes of the tropical ocean, however, which play an important role in seasonal climate extremes such as hurricanes, floods and droughts at the present day, are not well known for this period that led into the last glacial. Here we present a monthly resolved snapshot of reconstructed sea surface temperature in the tropical North Atlantic Ocean for 117.7±0.8 kyr ago, using coral Sr/Ca and d18O records. We find that temperature seasonality was similar to today, which is consistent with the orbital insolation forcing. Our coral and climate model results suggest that temperature seasonality of the tropical surface ocean is controlled mainly by orbital insolation changes during interglacials.

Processed CO2/O2 Float data from 4 deployments near Cape Verde Island (11/2010 - 06/2011)

In recent years, profiling floats, which form the basis of the successful international Argo observatory, are also being considered as platforms for marine biogeochemical research. This study showcases the utility of floats as a novel tool for combined gas measurements of CO2 partial pressure (pCO2) and O2. These float prototypes were equipped with a small-sized and submersible pCO2 sensor and an optode O2 sensor for highresolution measurements in the surface ocean layer. Four consecutive deployments were carried out during November 2010 and June 2011 near the Cape Verde Ocean Observatory (CVOO) in the eastern tropical North Atlantic. The profiling float performed upcasts every 31 h while measuring pCO2, O2, salinity, temperature, and hydrostatic pressure in the upper 200 m of the water column. To maintain accuracy, regular pCO2 sensor zeroings at depth and surface, as well as optode measurements in air, were performed for each profile. Through the application of data processing procedures (e.g., time-lag correction), accuracies of floatborne pCO2 measurements were greatly improved (10-15 µatm for the water column and 5 µatm for surface measurements). O2 measurements yielded an accuracy of 2 µmol/kg. First results of this pilot study show the possibility of using profiling floats as a platform for detailed and unattended observations of the marine carbon and oxygen cycle dynamics.

Zooplankton data of M83/1

Oxygen-deficient waters in the ocean, generally referred to as oxygen minimum zones (OMZ), are expected to expand as a consequence of global climate change. Poor oxygenation is promoting microbial loss of inorganic nitrogen (N) and increasing release of sediment-bound phosphate (P) into the water column. These intermediate water masses, nutrient-loaded but with an N deficit relative to the canonical N:P Redfield ratio of 16:1, are transported via coastal upwelling into the euphotic zone. To test the impact of nutrient supply and nutrient stoichiometry on production, partitioning and elemental composition of dissolved (DOC, DON, DOP) and particulate (POC, PON, POP) organic matter, three nutrient enrichment experiments were conducted with natural microbial communities in shipboard mesocosms, during research cruises in the tropical waters of the southeast Pacific and the northeast Atlantic. Maximum accumulation of POC and PON was observed under high N supply conditions, indicating that primary production was controlled by N availability. The stoichiometry of microbial biomass was unaffected by nutrient N:P supply during exponential growth under nutrient saturation, while it was highly variable under conditions of nutrient limitation and closely correlated to the N:P supply ratio, although PON:POP of accumulated biomass generally exceeded the supply ratio. Microbial N:P composition was constrained by a general lower limit of 5:1. Channelling of assimilated P into DOP appears to be the mechanism responsible for the consistent offset of cellular stoichiometry relative to inorganic nutrient supply and nutrient drawdown, as DOP build-up was observed to intensify under decreasing N:P supply. Low nutrient N:P conditions in coastal upwelling areas overlying O2-deficient waters seem to represent a net source for DOP, which may stimulate growth of diazotrophic phytoplankton. These results demonstrate that microbial nutrient assimilation and partitioning of organic matter between the particulate and the dissolved phase are controlled by the N:P ratio of upwelled nutrients, implying substantial consequences for nutrient cycling and organic matter pools in the course of decreasing nutrient N:P stoichiometry.