Ongoing transients in carbonate compensation

Uptake of anthropogenic CO2 is acidifying the oceans. Over the next 2000 years, this will modify the dissolution and preservation of sedimentary carbonate. By coupling new formulas for the positions of the calcite saturation horizon, zsat, the compensation depth, zcc, and the snowline, zsnow, to a biogeochemical model of the oceanic carbonate system, we evaluate how these horizons will change with ongoing ocean acidification. Our model is an extended Havardton-Bear-type box model, which includes novel kinetic descriptions for carbonate dissolution above, between, and below these critical depths. In the preindustrial ocean, zsat and zcc are at 3939 and 4750 m, respectively. When forced with the IS92a CO2 emission scenario, the model forecasts (1) that zsat will rise rapidly (“runaway” conditions) so that all deep water becomes undersaturated, (2) that zcc will also rise and over 1000 years will pass before it will be stabilized by the dissolution of previously deposited CaCO3, and (3) that zsnow will respond slowly to acidification, rising by ∼1150 m during a 2000 year timeframe. A further simplified model that equates the compensation and saturation depths produces quantitatively different results. Finally, additional feedbacks due to acidification on calcification and increased atmospheric CO2 on organic matter productivity strongly affect the positions of the compensation horizons and their dynamics.

Upwelling rates for the equatorial Pacific Ocean derived from the bomb 14C distribution

A north-south cross section of bomb-produced radiocarbon (14C) in the upper 1000 m of the central equatorial Pacific Ocean (CEP) was measured in April, 1979 during Leg 3 of the NORPAX shuttle experiment. The 14C shows an equatorial mixed layer depletion of ∼40‰ compared to subtropical surface waters. Upwelling of deeper, 14C depleted water maintains this minimum. Two subsurface tongues of high 14C water, found north and south of the equator, are associated with high salinity water and probably result from exchange with subtropical surface water. The continued increase in mixed layer 14C levels in the CEP (up to 1979) indicates the importance of 14C input from these subsurface 14C maxima. Equatorward meridional advection resulting from geostrophic flow is the predominant supply of water upwelling at the equator and controls the 14C distribution in the CEP. The results of multi-layer mixing model calculations indicate an upwelling transport rate of 47 Sverdrups (5S–4N) and a maximum depth of upwelling of 225 m (σ0 = 26.5). These equatorial circulation characteristics explain the 14C, ΣCO2, oxygen, salinity and tritium distributions measured during Leg 3. The time history of mixed layer bomb 14C concentrations in the CEP indicate an exchange time of 4–6 years between the subtropical and equatorial surface oceans

An additional deep-water mass in Drake Passage as revealed by 3He data

We present 3He data froma repeat section across Drake Passage, fromthree sections off the South American continent in the Pacific, at 28?S, 35?S, and 43?S, and fromthree sections in the Atlantic, eastward of the Malvinas, close to 35?W, and near the Greenwich Meridian. In Drake Passage, a distinct high-3He signal is observed that is centered just above the boundary of the Lower and the Upper Circumpolar Deep Water (LCDW, UCDW), and is concentrated towards the northern continental slope. 3He concentrations in the Antarctic Circumpolar Current (ACC) upstream of Drake Passage (World Ocean Circulation Experiment section P19 at 88?W) are markedly lower than those found in Drake Passage, and a regional source of primordial helium in the path of the ACC that might cause the high-3He feature can be ruled out. We explain the feature by addition of high-3He waters present at the 43?S Pacific section. This supports a previous, similar interpretation of a low-salinity anomaly in Drake Passage (Naveira Garabato et al., Deep- Sea Research I 49 (2002) 681), that is strongly related to the high-3He feature. Employing multiparameter water mass analysis (including 3He as a parameter), we find that deep waters as met at the 43?S Pacific section, flowing south along the South American continental slope, contribute substantially to the ACC waters in Drake Passage (fractions exceed 50% locally). Lesser, but laterally more extended contributions are found east of the Malvinas, and still smaller ones are present at 35?W and at the Greenwich Meridian. Using velocity measurements from one of the two Drake Passage sections, we estimate the volume transport of these waters to be 7.071.2 Sv, but the average transport may be somewhat lower as the other realization had a less pronounced signal. The enhanced 3He signature in Drake Passage is essentially confined north of the Polar Front. Further downstreamthe signature crosses this front, to the extent that at 35?W the contributions south and north of it are of similar magnitude. At the same time, the 3He levels north of the front are reduced due to a substantial admixture of low-3He North Atlantic Deep Water, such that 3He becomes highest south of the front. The flow of Southeast Pacific deep slope waters entering the ACC constitutes the predominant exit pathway of the primordial helium released in the deep Pacific, and represents a considerable fraction of the deep water return flow fromthe Pacific into the ACC. Therefore and also because the density range of the added deep slope waters is intermediate between those of UCDW and LCDW, they must be considered a distinct water mass. r 2003 Elsevier Ltd. All rights reserved

Constraints on mantle ^3He fluxes and deep-sea circulation from an oceanic general circulation model

We have simulated the steady-state distribution of helium in the deep sea to investigate the magnitude and spatial and temporal variability of mantle degassing and to characterize deep-sea circulation and ventilation. The simulation was produced by linking a simple source function for helium injected at mid-ocean ridges with an oceanic general circulation model (GCM). By assuming that the flux of mantle helium is linearly proportional to the seafloor spreading rate and by using previous estimates for the total flux of mantle helium into the oceans, the GCM yields an oceanic ^3He distribution which is in qualitative agreement with observations both in overall magnitude and in general distribution. This provides new evidence that the flux of mantle ^3He into the oceans is about 1000 mol/yr and that mid-ocean ridges are the dominant source of mantle helium. Although the match with observations is good in the Pacific and Indian Oceans, the simulated ^3He anomalies throughout the Atlantic Ocean are much higher than has been measured. Because the GCM is thought to reproduce Atlantic circulation reasonably well, this discrepancy suggests an error in the helium source function. Either helium injection is not a linear function of seafloor emplacement rate, or eruption and concomitant degassing are highly episodic at the slow spreading rates characteristic of the Mid-Atlantic Ridge (MAR). The latter explanation would imply minimal volcanic activity along the entire length of the MAR over the last few centuries. In addition to constraints on the degassing flux, our work provides information on the transport and ventilation of deep ocean waters and constrains the degree to which current GCMs can reproduce deep-water circulation patterns. While the results generally support the GCM's abyssal circulation, our simulation reveals regions of overly-intense lateral diffusion and upwelling in the model, particularly in the equatorial Pacific. Similarly, there appears to be insufficient production of He-ventilated bottom waters in the model Antarctic. These observations suggest that further refinement of the GCM abyssal circulation is required

Bioturbation artifacts in zero-age sediments

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 24 (2009): PA4212, doi:10.1029/2008PA001727.Most seafloor sediments are dated with radiocarbon, and the sediment is assumed to be zero-age (modern) when the signal of atmospheric testing of nuclear weapons is present (Fraction modern (Fm) > 1). Using a simple mass balance, we show that even with Fm > 1, half of the planktonic foraminifera at the seafloor can be centuries old, because of bioturbation. This calculation, and data from four core sites in the western North Atlantic indicate that, first, during some part of the Little Ice Age (LIA) there may have been more Antarctic Bottom Water than today in the deep western North Atlantic. Alternatively, bioturbation may have introduced much older benthic foraminifera into surface sediments. Second, paleo-based warming of Sargasso Sea surface waters since the LIA must lag the actual warming because of bioturbation of older and colder foraminifera.This work was funded in part by the Gary Comer Foundation and by NSF grant 0214144. A portion of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

Anatomy of a Dansgaard-Oeschger warming transition: High-resolution analysis of the North Greenland Ice Core Project ice core

Large and abrupt temperature oscillations during the last glacial period, known as Dansgaard‐Oeschger (DO) events, are clearly observed in the Greenland ice core record. Here we present a new high‐resolution chemical (2 mm) and stable isotope (20 mm) record from the North Greenland Ice Core Project (NGRIP) ice core at the onset of one of the most prominent DO events of the last glacial, DO‐8, observed ∼38,000 years ago. The unique, subannual‐resolution NGRIP record provides a true sequence of change during a DO warming with detailed annual layer counting of very high depth resolution geochemical measurements used to determine the exact duration of the transition. The continental ions, indicative of long‐range atmospheric loading and dustiness from East Asia, are the first to change, followed by the snow accumulation, the moisture source conditions, and finally the atmospheric temperature in Greenland. The sequence of events shows that atmospheric and oceanic source and circulation changes preceded the DO warming by several years

Tracing Noble Gas Radionuclides in the Environment

Trace analysis of radionuclides is an essential and versatile tool in modern science and technology. Due to their ideal geophysical and geochemical properties, long-lived noble gas radionuclides, in particular, 39Ar (t1/2 = 269 yr), 81Kr (t1/2 = 2.3x10^5 yr) and 85Kr (t1/2 = 10.8 yr), have long been recognized to have a wide range of important applications in Earth sciences. In recent years, significant progress has been made in the development of practical analytical methods, and has led to applications of these isotopes in the hydrosphere (tracing the flow of groundwater and ocean water). In this article, we introduce the applications of these isotopes and review three leading analytical methods: Low-Level Counting (LLC), Accelerator Mass Spectrometry (AMS) and Atom Trap Trace Analysis (ATTA)

Multivalent display of minimal Clostridium difficile glycan epitopes mimics antigenic properties of larger glycans

Synthetic cell-surface glycans are promising vaccine candidates against Clostridium difficile. The complexity of large, highly antigenic and immunogenic glycans is a synthetic challenge. Less complex antigens providing similar immune responses are desirable for vaccine development. Based on molecular-level glycan-antibody interaction analyses, we here demonstrate that the C. difficile surface polysaccharide-I (PS-I) can be resembled by multivalent display of minimal disaccharide epitopes on a synthetic scaffold that does not participate in binding. We show that antibody avidity as a measure of antigenicity increases by about five orders of magnitude when disaccharides are compared with constructs containing five disaccharides. The synthetic, pentavalent vaccine candidate containing a peptide T-cell epitope elicits weak but highly specific antibody responses to larger PS-I glycans in mice. This study highlights the potential of multivalently displaying small oligosaccharides to achieve antigenicity characteristic of larger glycans. The approach may result in more cost-efficient carbohydrate vaccines with reduced synthetic effort

A Human Development Framework for CO2 Reductions

Although developing countries are called to participate in CO2 emission reduction efforts to avoid dangerous climate change, the implications of proposed reduction schemes in human development standards of developing countries remain a matter of debate. We show the existence of a positive and time-dependent correlation between the Human Development Index (HDI) and per capita CO2 emissions from fossil fuel combustion. Employing this empirical relation, extrapolating the HDI, and using three population scenarios, the cumulative CO2 emissions necessary for developing countries to achieve particular HDI thresholds are assessed following a Development As Usual approach (DAU). If current demographic and development trends are maintained, we estimate that by 2050 around 85% of the world's population will live in countries with high HDI (above 0.8). In particular, 300Gt of cumulative CO2 emissions between 2000 and 2050 are estimated to be necessary for the development of 104 developing countries in the year 2000. This value represents between 20% to 30% of previously calculated CO2 budgets limiting global warming to 2{\deg}C. These constraints and results are incorporated into a CO2 reduction framework involving four domains of climate action for individual countries. The framework reserves a fair emission path for developing countries to proceed with their development by indexing country-dependent reduction rates proportional to the HDI in order to preserve the 2{\deg}C target after a particular development threshold is reached. Under this approach, global cumulative emissions by 2050 are estimated to range from 850 up to 1100Gt of CO2. These values are within the uncertainty range of emissions to limit global temperatures to 2{\deg}C.Comment: 14 pages, 7 figures, 1 tabl