Description: In a recent article, Lyle et al. (2005) used acoustic data to argue that 230Th-normalization overestimates sediment focusing in the equatorial Pacific by as much as an order of magnitude. They argue that lateral transport of 230Th in the water column results in much wider geographical variability in the vertical flux of scavenged 230Th than recognized by geochemists who have studied the behavior of this isotope in the water column. We disagree with this view and point to what we perceive as flaws in the arguments presented by these authors.

Description: Trace metals in the ocean act as both essential micro-nutrients and as toxins. There are relatively few multi-element studies of dissolved trace metals in the ocean, and none from the Gulf of Aqaba, Red Sea. This semi-enclosed basin surrounded by desert is a natural laboratory for studying the impact of atmospheric dry deposition of trace metals on the ocean surface. We have combined measurement of dissolved metals in seawater with measurements of the flux of metals associated with dry deposition. The total dissolved trace metal concentrations in Gulf of Aqaba water are generally higher (Fe, Cu, Zn, Co, Mn, Pb) or similar (Ni, Al, Cd, Mo) to those measured in the open North Atlantic Ocean. The concentrations of elements that are highly enriched in aerosols relative to Al (e.g. Cd, Pb, Zn and Cu) are not necessarily proportionally enriched in surface seawater when compared to Al, indicative of the high reactivity of these elements in seawater. Iron concentrations in the Gulf of Aqaba are high relative to Al, despite the fact that the aerosols are not more enriched in Fe relative to Al. There may be additional sources of dissolved iron to the Gulf of Aqaba, not associated with Al. Alternatively, intense photochemically-driven redox cycling may act to enhance Fe dissolution from aerosols, or may otherwise increase the lifetime of Fe in the water column, relative to Al. Copper concentrations in the Gulf of Aqaba are close to the value found to be a threshold for Cu toxicity in this region. A surface maximum in Cd:P is found in the Gulf of Aqaba, in contrast to the more typical surface minimum in this ratio observed in other locations. The surface maximum appears to be driven by atypically low uptake of Cd relative to P. A low Cd:P uptake ratio for this region is consistent with known environmental determinants of low Cd:P uptake, such as high concentrations of dissolved Zn and Fe, and a predominance of small phytoplankton including cyanobacteria. Highlights ► We measured dissolved trace metal concentrations in the Gulf of Aqaba four times. ► Iron concentrations are high relative to Al concentrations. ► Cu concentrations are close to the threshold for Cu toxicity in this region. ► A surface maximum in Cd:P is driven by unusually low uptake of Cd relative to PO4. ► This is consistent with the dominance of small phytoplankton and high Fe levels.

Description: We present the first comprehensive investigation of the concentrations, fluxes and sources of aerosol trace elements over the Gulf of Aqaba. We found that the mean atmospheric concentrations of crustally derived elements such as Al, Fe and Mn (1081, 683, and 16.7 ng m�3) are about 2–3 times higher than those reported for the neighboring Mediterranean area. This is indicative of the dominance of the mineral dust component in aerosols over the Gulf. Anthropogenic impact was lower in comparison to the more heavily populated areas of the Mediterranean. During the majority of time (69%) the air masses over the Gulf originated from Europe or Mediterranean Sea areas delivering anthropogenic components such as Cu, Cd, Ni, Zn, and P. Airflows derived from North Africa in contrast contained the highest concentrations of Al, Fe, and Sr but generally lower Cu, Cd, Ni, Zn, and P. Relatively high Pb, Ni, and V were found in the local and Arabian airflows suggesting a greater influence of local emission of fuel burning. We used the data and the measured trace metal seawater concentrations to calculate residence times of dissolved trace elements in the upper 50 m surface water of the Gulf (with respect to atmospheric input) and found that the residence times for most elements are in the range of 5–37 years while Cd and V residence times are longer.

Description: Chemical and biological sensor technologies have advanced rapidly in the past five years. Sensors that require low power and operate for multiple years are now available for oxygen, nitrate, and a variety of bio-optical properties that serve as proxies for important components of the carbon cycle (e.g., particulate organic carbon). These sensors have all been deployed successfully for long periods, in some cases more than three years, on platforms such as profiling floats or gliders. Technologies for pH, pCO2, and particulate inorganic carbon are maturing rapidly as well. These sensors could serve as the enabling technology for a global biogeochemical observing system that might operate on a scale comparable to the current Argo array. Here, we review the scientific motivation and the prospects for a global observing system for ocean biogeochemistry.

Description: The magnitude and spatial variability of millennial-scale changes in Southern Ocean temperature during Marine Isotope Stage 3 (MIS-3) are poorly constrained. Here we present a compilation of 14 previously published high-resolution sea surface temperature records from 30°S to 70°S. At each site we re-calibrate radiocarbon dates and synchronise records with the Antarctic Ice Core Chronology 2012 and then determine the presence, amplitude and duration of millennial-scale warming that correspond to Antarctic Isotope Maximum (AIM) events. Individual sediment cores recorded warming during an average of 7 out of 10 AIM events. These warming events were then used as tie-points to refine the age models at each site before combining records into Southern Ocean and basin-wide averaged temperature anomaly records or “stacks”. The resulting stacks of Southern Ocean, Atlantic and Indo-Pacific Ocean basin temperature anomalies show a signal consistent with AIM events during MIS-3. Stacked amplitudes of warming are also positively correlated with the amplitude of temperature increases in the EPICA Dome C ice core (n = 10, r2 = 0.65, p = 0.001). Additionally, rates of warming in the Southern Ocean are consistent across all warming events. These findings are consistent with the thermal seesaw hypothesis, where a reduction in Atlantic Meridional Overturning Circulation causes decreased cross-equatorial heat transport in the Atlantic resulting in heat accumulation in the south. Our results solidify the link between northern high latitude climate variability, Antarctic temperature and Southern Ocean surface temperature variability during MIS-3. This compilation of records with updated age models tied to the same ice core provides the first basin-scale synthesis of the sea surface temperature changes in the Southern Ocean during the rapid climate changes of MIS-3.

Description: Model intercomparison studies of coupled carbon-climate simulations have the potential to improve our understanding of the processes explaininEg the pCO2 drawdown at the Last Glacial Maximum (LGM) and to identify related model biases. Models participating in the Paleoclimate Modeling Intercomparison Project (PMIP) now frequently include the carbon cycle. The ongoing PMIP-carbon project provides the first opportunity to conduct multimodel comparisons of simulated carbon content for the LGM time window. However, such a study remains challenging due to differing implementation of ocean boundary conditions (e.g., bathymetry and coastlines reflecting the low sea level) and to various associated adjustments of biogeochemical variables (i.e., alkalinity, nutrients, dissolved inorganic carbon). After assessing the ocean volume of PMIP models at the pre-industrial and LGM, we investigate the impact of these modeling choices on the simulated carbon at the global scale, using both PMIP-carbon model outputs and sensitivity tests with the iLOVECLIM model. We show that the carbon distribution in reservoirs is significantly affected by the choice of ocean boundary conditions in iLOVECLIM. In particular, our simulations demonstErate a ~ 250 GtC effect of an alkalinity adjustment on carbon sequestration in the ocean. Finally, we observe that PMIP-carbon models with a freely evolving CO2 and no additional glacial mechanisms do not simulaEte the pCO2 drawdown at the LGM (with concentrations as high as 313, 331, and 315 ppm), especially if they use a low ocean volume. Our findings suggest that great care should be taken on accounting for large bathymetry changes in models including the carbon cycle.