A deep-sea coral record of North Atlantic radiocarbon through the Younger Dryas: Evidence for intermediate water/deepwater reorganization

Our record of Younger Dryas intermediate-depth seawater Δ^(14)C from North Atlantic deep-sea corals supports a link between abrupt climate change and intermediate ocean variability. Our data show that northern source intermediate water (∼1700 m) was partially replaced by (14)^C-depleted southern source water at the onset of the event, consistent with a reduction in the rate of North Atlantic Deep Water formation. This transition requires the existence of large, mobile gradients of Δ^(14)C in the ocean during the Younger Dryas. The Δ^(14)C water column profile from Keigwin (2004) provides direct evidence for the presence of one such gradient at the beginning of the Younger Dryas (∼12.9 ka), with a 100‰ offset between shallow (<∼2400 m) and deep water. Our early Younger Dryas data are consistent with this profile and also show a Δ^(14)C inversion, with 35‰ more enriched water at ∼2400 m than at ∼1700 m. This feature is probably the result of mixing between relatively well ^(14)C ventilated northern source water and more poorly ^(14)C ventilated southern source intermediate water, which is slightly shallower. Over the rest of the Younger Dryas our intermediate water/deepwater coral Δ^(14)C data gradually increase, while the atmosphere Δ^(14)C drops. For a very brief interval at ∼12.0 ka and at the end of the Younger Dryas (11.5 ka), intermediate water Δ^(14)C (∼1200 m) approached atmospheric Δ14C. These enriched Δ^(14)C results suggest an enhanced initial Δ^(14)C content of the water and demonstrate the presence of large lateral Δ^(14)C gradients in the intermediate/deep ocean in addition to the sharp vertical shift at ∼2500 m. The transient Δ^(14)C enrichment at ∼12.0 ka occurred in the middle of the Younger Dryas and demonstrates that there is at least one time when the intermediate/deep ocean underwent dramatic change but with much smaller effects in other paleoclimatic records

Conservative, special-relativistic smoothed particle hydrodynamics

We present and test a new, special-relativistic formulation of Smoothed Particle Hydrodynamics (SPH). Our approach benefits from several improvements with respect to earlier relativistic SPH formulations. It is self-consistently derived from the Lagrangian of an ideal fluid and accounts for special-relativistic "grad-h terms". In our approach, we evolve the canonical momentum and the canonical energy per baryon and thus circumvent some of the problems that have plagued earlier formulations of relativistic SPH. We further use a much improved artificial viscosity prescription which uses the extreme local eigenvalues of the Euler equations and triggers selectively on a) shocks and b) velocity noise. The shock trigger accurately monitors the relative density slope and uses it to fine-tune the amount of artificial viscosity that is applied. This procedure substantially sharpens shock fronts while still avoiding post-shock noise. If not triggered, the viscosity parameter of each particle decays to zero. None of these viscosity triggers is specific to special relativity, both could also be applied in Newtonian SPH. The performance of the new scheme is explored in a large variety of benchmark tests where it delivers excellent results. Generally, the grad-h terms deliver minor, though worthwhile, improvements. The scheme performs close to perfect in supersonic advection tests, but also in strong relativistic shocks, usually considered a particular challenge for SPH, the method yields convincing results. For example, due to its perfect conservation properties, it is able to handle Lorentz-factors as large as $\gamma= 50 \; 000$ in the so-called wall shock test. Moreover, we find convincing results in a rarely shown, but challenging test that involves so-called relativistic simple waves and also in multi-dimensional shock tube tests.Comment: 39 pages, 19 figures, Journal of Computational Physics in press, reference upate

Production and temperature sensitivity of long chain alkenones in the cultured haptophyte Pseudoisochrysis paradoxa

The alkenone unsaturation index (U&lt;sub&gt;37&lt;/sub&gt;&lt;sup&gt;K&lt;/sup&gt; or U&lt;sub&gt;37&lt;/sub&gt;&lt;sup&gt;K′&lt;/sup&gt;) serves as a critical tool for reconstructing temperature in marine environments. Lacustrine haptophyte algae are genetically distinct from their ubiquitous and well studied marine counterparts, and the unknown species-specific genetic imprints on long chain alkenone production by lacustrine species have hindered the widespread application of the U37&lt;sup&gt;K&lt;/sup&gt; temperature proxy to lake sediment records. The haptophyte Pseudoisochrysis paradoxa produces alkenones but its U37&lt;sup&gt;K&lt;/sup&gt; calibration has never been determined. It has an alkenone fingerprint abundant in tetraunsaturated alkenones, a hallmark of lacustrine environments. We present here the first calibration of the U37&lt;sup&gt;K&lt;/sup&gt; index to temperature for a culture of P. paradoxa. We found that the U37&lt;sup&gt;K&lt;/sup&gt; index accurately captured the alkenone response to temperature whereas the U37&lt;sup&gt;K′&lt;/sup&gt; index failed to do so, with U37&lt;sup&gt;K′&lt;/sup&gt; values below 0.08 projecting to two different temperature values. Our results add a fifth species-specific U37&lt;sup&gt;K&lt;/sup&gt; calibration and provide another line of evidence that different haptophyte species require different U37&lt;sup&gt;K&lt;/sup&gt; calibrations. The findings also highlight the necessary inclusion of the C&lt;sub&gt;37:4&lt;/sub&gt; alkenone when reconstructing temperatures from P. paradoxa-derived alkenone records

Djehuty: A Code for Modeling Whole Stars in Three Dimensions

The DJEHUTY project is an intensive effort at the Lawrence Livermore National Laboratory (LLNL) to produce a general purpose 3-D stellar structure and evolution code to study dynamic processes in whole stars.Comment: 2 pages, IAU coll. 18

Climate system modeling on massively parallel systems: LDRD Project 95-ERP-47 final report

Global warming, acid rain, ozone depletion, and biodiversity loss are some of the major climate-related issues presently being addressed by climate and environmental scientists. Because unexpected changes in the climate could have significant effect on our economy, it is vitally important to improve the scientific basis for understanding and predicting the earth`s climate. The impracticality of modeling the earth experimentally in the laboratory together with the fact that the model equations are highly nonlinear has created a unique and vital role for computer-based climate experiments. However, today`s computer models, when run at desired spatial and temporal resolution and physical complexity, severely overtax the capabilities of our most powerful computers. Parallel processing offers significant potential for attaining increased performance and making tractable simulations that cannot be performed today. The principal goals of this project have been to develop and demonstrate the capability to perform large-scale climate simulations on high-performance computing systems (using methodology that scales to the systems of tomorrow), and to carry out leading-edge scientific calculations using parallelized models. The demonstration platform for these studies has been the 256-processor Cray-T3D located at Lawrence Livermore National Laboratory. Our plan was to undertake an ambitious program in optimization, proof-of-principle and scientific study. These goals have been met. We are now regularly using massively parallel processors for scientific study of the ocean and atmosphere, and preliminary parallel coupled ocean/atmosphere calculations are being carried out as well. Furthermore, our work suggests that it should be possible to develop an advanced comprehensive climate system model with performance scalable to the teraflops range. 9 refs., 3 figs

Low reservoir ages for the surface ocean from mid-Holocene Florida corals

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 23 (2008): PA2209, doi:10.1029/2007PA001527.The 14C reservoir age of the surface ocean was determined for two Holocene periods (4908–4955 and 3008–3066 calendar (cal) B.P.) using U/Th-dated corals from Biscayne National Park, Florida, United States. We found that the average reservoir ages for these two time periods (294 ± 33 and 291 ± 27 years, respectively) were lower than the average value between A.D. 1600 and 1900 (390 ± 60 years) from corals. It appears that the surface ocean was closer to isotopic equilibrium with CO2 in the atmosphere during these two time periods than it was during recent times. Seasonal δ 18O measurements from the younger coral are similar to modern values, suggesting that mixing with open ocean waters was indeed occurring during this coral's lifetime. Likely explanations for the lower reservoir age include increased stratification of the surface ocean or increased Δ14C values of subsurface waters that mix into the surface. Our results imply that a more correct reservoir age correction for radiocarbon measurements of marine samples in this location from the time periods ∼3040 and ∼4930 cal years B.P. is ∼292 ± 30 years, less than the canonical value of 404 ± 20 years.NSF Chemical Oceanography program provided monetary support under grants OCE-9711326, OCE-0137207, and OCE-0551940 (to ERMD)

Integrated astrophysical modeling

In this project, we have developed prototype techniques for defining and extending a variety of astrophysical modeling capabilities, including those involving multidimensional hydrodynamics, complex transport, and flexibly-coupled equation-of state and nuclear reaction networks. As expected, this project is having both near-term payoffs in understanding complex astrophysical phenomena, as well as significant spin-offs in terms of people and ideas to related ASCI code efforts. Most of our work in the first part of this project was focused on the modularization, extension, and initial integration of 4 previously separate and incommensurate codes: the stellar evolution/explosion code KEPLER; the non-LTE spectral line transport code, EDDINGTON, used for modeling supernovae spectra; the 3-D smooth particle hydro code, PIP; and the discontinuous-finite-element, 3D hydro module from the lCF3D code

Acclimation of<i>E</i><i>miliania huxleyi</i>(1516) to nutrient limitation involves precise modification of the proteome to scavenge alternative sources of N and P

Limitation of marine primary production by the availability of nitrogen or phosphorus is common. Emiliania huxleyi, a ubiquitous phytoplankter that plays key roles in primary production, calcium carbonate precipitation and production of dimethyl sulfide, often blooms in mid-latitude at the beginning of summer when inorganic nutrient concentrations are low. To understand physiological mechanisms that allow such blooms, we examined how the proteome of E. huxleyi (strain 1516) responds to N and P limitation. We observed modest changes in much of the proteome despite large physiological changes (e.g. cellular biomass, C, N and P) associated with nutrient limitation of growth rate. Acclimation to nutrient limitation did however involve significant increases in the abundance of transporters for ammonium and nitrate under N limitation and for phosphate under P limitation. More notable were large increases in proteins involved in the acquisition of organic forms of N and P, including urea and amino acid/polyamine transporters and numerous C-N hydrolases under N limitation and a large upregulation of alkaline phosphatase under P limitation. This highly targeted reorganization of the proteome towards scavenging organic forms of macronutrients gives unique insight into the molecular mechanisms that underpin how E. huxleyi has found its niche to bloom in surface waters depleted of inorganic nutrients

Comprehensive climate system modeling on massively parallel computers

A better understanding of both natural and human induced changes to the Earth`s climate is necessary for policy makers to make informed decisions regarding energy usage and other greenhouse gas producing activities. To achieve this, substantial increases in the sophistication of climate models are required. Coupling between the climate subsystems of the atmosphere, oceans, cryosphere and biosphere is only now beginning to be explored in global models. the enormous computational expenses of such models is one significant factor limiting progress. A comprehensive climate system model targeted to distributed memory massively parallel processing (MPP) computers is under development at Lawrence Livermore National Laboratory. This class of computers promises the computational power to permit the timely execution of climate models of substantially more sophistication than current generation models. Our strategy for achieving high performance on large numbers of processors is to exploit the multiple layers of parallelism naturally contained within highly coupled global climate models. The centerpiece of this strategy is the concurrent execution of multiple independently parallelized components of the climate system model. This methodology allows the assignment of an arbitrary number of processors to each of the major climate subsystems. Hence, a higher total number of processors may be efficiently used. Furthermore, load imbalances arising from the coupling of submodels may be minimized by adjusting the distribution of processors among the submodels