@article{HaugSigman2009, author = {Haug, Gerald H. and Sigman, Daniel M.}, title = {Palaeoceanography : polar twins}, issn = {1752-0894}, doi = {10.1038/Ngeo423}, year = {2009}, abstract = {Ice ages in the North Pacific Ocean and the Southern Ocean were marked by low productivity. Accumulating evidence indicates that strong stratification restricted the supply of nutrients from the deep ocean to the algae of the sunlit surface in these regions.}, language = {en} } @article{SigmanDiFioreHainetal.2009, author = {Sigman, Daniel M. and DiFiore, Peter J. and Hain, Mathis P. and Deutsch, Curtis and Karl, David M.}, title = {Sinking organic matter spreads the nitrogen isotope signal of pelagic denitrification in the North Pacific}, issn = {0094-8276}, doi = {10.1029/2008gl035784}, year = {2009}, abstract = {Culture studies of denitrifying bacteria predict that denitrification will generate equivalent gradients in the delta N-15 and delta O-18 of deep ocean nitrate. A depth profile of nitrate isotopes from the Hawaii Ocean Time-series Station ALOHA shows less of an increase in delta O-18 than in delta N-15 as one ascends from abyssal waters into the denitrification-impacted mid-depth waters. A box model of the ocean nitrate N and O isotopes indicates that this is the effect of the low latitude nitrate assimilation/regeneration cycle: organic N sinking out of the surface spreads the high-delta N-15 signal of pelagic denitrification into waters well below and beyond the suboxic zone, whereas the nitrate delta O-18 signal of denitrification can only be transmitted by circulation in the interior.}, language = {en} } @article{SigmanDiFioreHainetal.2009, author = {Sigman, Daniel M. and DiFiore, Peter J. and Hain, Mathis P. and Deutsch, Curtis and Wang, Yi and Karl, David M. and Knapp, Angela N. and Lehmann, Moritz F. and Pantoja, Silvio}, title = {The dual isotopes of deep nitrate as a constraint on the cycle and budget of oceanic fixed nitrogen}, issn = {0967-0637}, doi = {10.1016/j.dsr.2009.04.007}, year = {2009}, abstract = {We compare the output of an 18-box geochemical model of the ocean with measurements to investigate the controls on both the mean values and variation of nitrate delta N-15 and delta O-18 in the ocean interior. The delta O-18 of nitrate is our focus because it has been explored less in previous work. Denitrification raises the delta N-15 and delta O-18 of mean ocean nitrate by equal amounts above their input values for N-2 fixation (for delta N-15) and nitrification (for delta O-18), generating parallel gradients in the delta N-15 and delta O-18 of deep ocean nitrate. Partial nitrate assimilation in the photic zone also causes equivalent increases in the delta N-15 and delta O-18 of the residual nitrate that can be transported into the interior. However, the regeneration and nitrification of sinking N can be said to decouple the N and O isotopes of deep ocean nitrate, especially when the sinking N is produced in a low latitude region, where nitrate consumption is effectively complete. The delta N-15 of the regenerated nitrate is equivalent to that originally consumed, whereas the regeneration replaces nitrate previously elevated in delta O-18 due to denitrification or nitrate assimilation with nitrate having the delta O-18 of nitrification. This lowers the delta O-18 of mean ocean nitrate and weakens nitrate delta O-18 gradients in the interior relative to those in delta N-15. This decoupling is characterized and quantified in the box model, and agreement with data shows its clear importance in the real ocean. At the same time, the model appears to generate overly strong gradients in both delta O-18 and delta N-15 within the ocean interior and a mean ocean nitrate delta O-18 that is higher than measured. This may be due to, in the model, too strong an impact of partial nitrate assimilation in the Southern Ocean on the delta N-15 and delta O-18 of preformed nitrate and/or too little cycling of intermediate-depth nitrate through the low latitude photic zone.}, language = {en} } @article{JaccardGalbraithSigmanetal.2009, author = {Jaccard, Samuel Laurent and Galbraith, Eric D. and Sigman, Daniel M. and Haug, Gerald H. and Francois, Roger and Pedersen, Thomas F. and Dulski, Peter and Thierstein, Hans R.}, title = {Subarctic Pacific evidence for a glacial deepening of the oceanic respired carbon pool}, issn = {0012-821X}, doi = {10.1016/j.epsl.2008.10.017}, year = {2009}, abstract = {Measurements of benthic foraminiferal cadmium:calcium (Cd/Ca) have indicated that the glacial-interglacial change in deep North Pacific phosphate (PO4) concentration was minimal which has been taken by some, workers as a sign that the biological pump did not store more carbon in the deep glacial ocean. Here we present sedimentary redox- sensitive trace metal records from Ocean Drilling Program (ODP) Site 882 (NW subarctic Pacific, water depth 3244 m) to make inferences about changes in deep North Pacific oxygenation and thus respired carbon storage - over the past 150,000 yr. These observations are complemented with biogenic barium and opal measurements as indicators for past organic carbon export to separate the influences of deep-water oxygen concentration and sedimentary organic carbon respiration on the redox state of the sediment. Our results suggest that the deep subarctic Pacific water mass was deleted in ox en during glacial maxima, though it was not anoxic. We reconcile our results with the existing benthic foraminiferal Cd/Ca by invoking a decrease in the fraction of the deep ocean nutrient inventory that was preformed, rather than remineralized. This change would have corresponded to an increase in the deep Pacific storage of respired carbon, which Would have lowered atmospheric carbon dioxide (CO2) by sequestering CO2 away from the atmosphere and by increasing ocean alkalinity through a transient dissolution event in the deep sea. The magnitude of change in preformed nutrients suggested by the North Pacific data Would have accounted for a majority of the observed decrease in glacial atmospheric PCO2.}, language = {en} } @article{RenSigmanMeckleretal.2009, author = {Ren, Haojia and Sigman, Daniel M. and Meckler, Anna Nele and Plessen, Birgit and Robinson, Rebecca S. and Rosenthal, Yair and Haug, Gerald H.}, title = {Foraminiferal isotope evidence of reduced nitrogen fixation in the Ice Age Atlantic ocean}, issn = {0036-8075}, doi = {10.1126/science.1165787}, year = {2009}, abstract = {Fixed nitrogen ( N) is a limiting nutrient for algae in the low- latitude ocean, and its oceanic inventory may have been higher during ice ages, thus helping to lower atmospheric CO2 during those intervals. In organic matter within planktonic foraminifera shells in Caribbean Sea sediments, we found that the N-15/N-14 ratio from the last ice age is higher than that from the current interglacial, indicating a higher nitrate N-15/N-14 ratio in the Caribbean thermocline. This change and other species- specific differences are best explained by less N fixation in the Atlantic during the last ice age. The fixation decrease was most likely a response to a known ice age reduction in ocean N loss, and it would have worked to balance the ocean N budget and to curb ice age- interglacial change in the N inventory.}, language = {en} } @article{SigmanHainHaug2010, author = {Sigman, Daniel M. and Hain, Mathis P. and Haug, Gerald H.}, title = {The polar ocean and glacial cycles in atmospheric CO2 concentration}, issn = {0028-0836}, doi = {10.1038/Nature09149}, year = {2010}, abstract = {Global climate and the atmospheric partial pressure of carbon dioxide (p(CO2atm)) are correlated over recent glacial cycles, with lower p(CO2atm) during ice ages, but the causes of the p(CO2atm) changes are unknown. The modern Southern Ocean releases deeply sequestered CO2 to the atmosphere. Growing evidence suggests that the Southern Ocean CO2 'leak' was stemmed during ice ages, increasing ocean CO2 storage. Such a change would also have made the global ocean more alkaline, driving additional ocean CO2 uptake. This explanation for lower ice-age p(CO2atm), if correct, has much to teach us about the controls on current ocean processes.}, language = {en} } @article{BrunelleSigmanJaccardetal.2010, author = {Brunelle, Brigitte G. and Sigman, Daniel M. and Jaccard, Samuel Laurent and Keigwin, Lloyd D. and Plessen, Birgit and Schettler, Georg and Cook, Mea S. and Haug, Gerald H.}, title = {Glacial/interglacial changes in nutrient supply and stratification in the western subarctic North Pacific since the penultimate glacial maximum}, issn = {0277-3791}, doi = {10.1016/j.quascirev.2010.03.010}, year = {2010}, abstract = {In piston cores from the open subarctic Pacific and the Okhotsk Sea, diatom-bound delta N-15 (delta N-15(db)), biogenic opal, calcium carbonate, and barium were measured from coretop to the previous glacial maximum (MIS 6). Glacial intervals are generally characterized by high delta N-15(db) (similar to 8 parts per thousand) and low productivity, whereas interglacial intervals have a lower delta N-15(db) (5.7-6.3 parts per thousand) and indicate high biogenic productivity. These data extend the regional swath of evidence for nearly complete surface nutrient utilization during glacial maxima, consistent with stronger upper water column stratification throughout the subarctic region during colder intervals. An early deglacial decline in delta N-15(db) of 2 parts per thousand at similar to 17.5 ka, previously observed in the Bering Sea, is found here in the open subarctic Pacific record and arguably also in the Okhotsk, and a case can be made that a similar decrease in delta N-15(db) occurred in both regions at the previous deglaciation as well. The early deglacial delta N-15(db) decrease, best explained by a decrease in surface nutrient utilization, appears synchronous with southern hemisphere-associated deglacial changes and with the Heinrich 1 event in the North Atlantic. This delta N-15(db) decrease may signal the initial deglacial weakening in subarctic North Pacific stratification and/or a deglacial increase in shallow subsurface nitrate concentration. If the former, it would be the North Pacific analogue to the increase in vertical exchange inferred for the Southern Ocean at the time of Heinrich Event 1. In either case, the lack of any clear change in paleoproductivity proxies during this interval would seem to require an early deglacial decrease in the iron-to-nitrate ratio of subsurface nutrient supply or the predominance of light limitation of phytoplankton growth during the deglaciation prior to Bolling-Allerod warming.}, language = {en} } @article{MartinezGarciaRosellMeleJaccardetal.2011, author = {Martinez-Garcia, Alfredo and Rosell-Mele, Antoni and Jaccard, Samuel L. and Geibert, Walter and Sigman, Daniel M. and Haug, Gerald H.}, title = {Southern Ocean dust-climate coupling over the past four million years}, series = {Nature : the international weekly journal of science}, volume = {476}, journal = {Nature : the international weekly journal of science}, number = {7360}, publisher = {Nature Publ. Group}, address = {London}, issn = {0028-0836}, doi = {10.1038/nature10310}, pages = {312 -- U141}, year = {2011}, abstract = {Dust has the potential to modify global climate by influencing the radiative balance of the atmosphere and by supplying iron and other essential limiting micronutrients to the ocean(1,2). Indeed, dust supply to the Southern Ocean increases during ice ages, and 'iron fertilization' of the subantarctic zone may have contributed up to 40 parts per million by volume (p. p. m. v.) of the decrease (80-100 p. p. m. v.) in atmospheric carbon dioxide observed during late Pleistocene glacial cycles(3-7). So far, however, the magnitude of Southern Ocean dust deposition in earlier times and its role in the development and evolution of Pleistocene glacial cycles have remained unclear. Here we report a high-resolution record of dust and iron supply to the Southern Ocean over the past four million years, derived from the analysis of marine sediments from ODP Site 1090, located in the Atlantic sector of the subantarctic zone. The close correspondence of our dust and iron deposition records with Antarctic ice core reconstructions of dust flux covering the past 800,000 years (refs 8, 9) indicates that both of these archives record large-scale deposition changes that should apply to most of the Southern Ocean, validating previous interpretations of the ice core data. The extension of the record beyond the interval covered by the Antarctic ice cores reveals that, in contrast to the relatively gradual intensification of glacial cycles over the past three million years, Southern Ocean dust and iron flux rose sharply at the Mid-Pleistocene climatic transition around 1.25 million years ago. This finding complements previous observations over late Pleistocene glacial cycles(5,8,9), providing new evidence of a tight connection between high dust input to the Southern Ocean and the emergence of the deep glaciations that characterize the past one million years of Earth history.}, language = {en} } @article{WolffHaugTimmermannetal.2011, author = {Wolff, Christian Michael and Haug, Gerald H. and Timmermann, Axel and Damste, Jaap S. Sinninghe and Brauer, Achim and Sigman, Daniel M. and Cane, Mark A. and Verschuren, Dirk}, title = {Reduced interannual rainfall variability in East Africa during the last Ice Age}, series = {Science}, volume = {333}, journal = {Science}, number = {6043}, publisher = {American Assoc. for the Advancement of Science}, address = {Washington}, issn = {0036-8075}, doi = {10.1126/science.1203724}, pages = {743 -- 747}, year = {2011}, abstract = {Interannual rainfall variations in equatorial East Africa are tightly linked to the El Nino Southern Oscillation (ENSO), with more rain and flooding during El Nino and droughts in La Nina years, both having severe impacts on human habitation and food security. Here we report evidence from an annually laminated lake sediment record from southeastern Kenya for interannual to centennial-scale changes in ENSO-related rainfall variability during the last three millennia and for reductions in both the mean rate and the variability of rainfall in East Africa during the Last Glacial period. Climate model simulations support forward extrapolation from these lake sediment data that future warming will intensify the interannual variability of East Africa's rainfall.}, language = {en} } @article{HainSigmanHaug2011, author = {Hain, Mathis P. and Sigman, Daniel M. and Haug, Gerald H.}, title = {Shortcomings of the isolated abyssal reservoir model for deglacial radiocarbon changes in the mid-depth Indo-Pacific Ocean}, series = {Geophysical research letters}, volume = {38}, journal = {Geophysical research letters}, number = {6}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0094-8276}, doi = {10.1029/2010GL046158}, pages = {6}, year = {2011}, abstract = {Severely negative Delta(14)C anomalies from the mid-depth Pacific and the Arabian Sea have been taken as support for the hypothesized deglacial release of a previously isolated, extremely (14)C-deplete deep ocean carbon reservoir. We report box model simulations that cast doubt on both the existence of the hypothesized deep reservoir and its ability to explain the mid-depth Delta(14)C anomalies. First, the degree of ice age isolation needed to substantially reduce the deep Delta(14)C of the deep reservoir causes anoxia and the trapping of alkalinity from CaCO(3) dissolution, the latter increasing atmospheric CO(2). Second, even with a completely (14)C-free deep reservoir, achieving the mid-depth Delta(14)C anomalies of observed duration requires ad hoc stifling of aspects of deep circulation to prevent rapid dissipation of the anomalous (14)C-free carbon to the rest of the ocean and the atmosphere. We suggest that the mid-depth anomalies do not record basin-scale Delta(14)C changes but are instead local phenomena.}, language = {en} }