PP11C-01 INVITED
Dynamic Paleoceanographic History of the Eastern Tropical Pacific – Is the confusion clearing?
In spite of the importance of the ENSO-sensitive region of the eastern tropical Pacific as an indicator of mechanisms tropical climate change, reconstructing the paleoceanographic history here has provoked controversy and confusion. Various authors claim that the glacial system is more like La Nina, or more like El Nino, or with little temperature change, or that it was much colder with links to the Antarctic, or the North Atlantic, or that the ITCZ moved Southward, or Northward, or not at all. Similar controversy has accompanied reconstruction of paleoproductivity, biogenic fluxes, and other fundamental properties, as well as with the timing and mechanisms of transient changes during the deglacial transition, and inferences about mechanisms (upwelling, advection, preformed properties of source waters, radiative forcing and greenhouse gases, etc). Models disagree too. Why is this place so confusing? We review some of this controversy, using multiple paleo proxies, and attempt to outline a possible solution including new data. We think the difficulty reflects biases in all the various paleo proxies related to interaction among oceanic processes (for example, productivity, pycnocline depth, and temperature), in the ways the proxies are measured, and in modifications due to sediment diagenesis fueled by high organic rain in this biologically productive region. Work toward resolving these issues in this controversial area leads us to think about paleo proxies in new ways, and we hope it will lead to more precise and accurate ways of reconstructing past conditions that will provide useful for better reconstructing the rest of the world. Simply understanding the difference between glacial and interglacial states is not sufficient, as this area – as with many others – has substantial transient and millennial-scale variations superimposed on longer-term changes. Looking at the full spectrum of variability in both data and models yields important insights.
PP11C-02 INVITED
Antarctic Intermediate Water: Passive Responder or Climate Trigger?
The Atlantic meridional overturning circulation (MOC), the system of warm-to-cold water transformation that results in North Atlantic Deep Water (NADW) production and its southward flow at depth, is a major component of the climate system and a significant source of heat to the circum-North Atlantic region. Northward flowing Antarctic Intermediate Water (AAIW) is an important constituent of the water that renews NADW formation. Early during the last deglaciation, a massive North Atlantic iceberg discharge event (Heinrich Event 1; H1) disrupted the MOC, cooling the North Atlantic and warming the southern hemisphere. It has been proposed that this warming caused melting of Antarctic ice sheet ~14,600 years ago (Meltwater Pulse 1A; MWP-1A), reducing the density of AAIW, and triggering the resumption of NADW and the abrupt warming into the Bølling period. Here, we provide evidence that enhanced penetration of AAIW into the North Atlantic began more than 1000 years before the Bølling warming, synchronously with the end of H1. The contribution of NADW to the deep North Atlantic also increased at this time, suggesting that the enhanced northward penetration of AAIW was linked to its role as a supplier of NADW, as it is in the modern ocean. We propose that the MOC recovered at this time because surface stratification decreased with the disappearance of the fresh surface waters associated with H1. The early NADW/AAIW increase and accompanying heat release caused modest warming over Greenland, suggesting that the changes in the Atlantic water masses we document were linked to changes in the MOC.
PP11C-03
Subtropical Atlantic salinity variability and Atlantic meridional circulation during the last deglaciation
During the last deglaciation (21 to 10 kyr BP), freshening of the surface of the North Atlantic likely caused reductions in Atlantic meridional overturning circulation (AMOC); what forced subsequent resumptions remains poorly understood. Here we present three new deglacial sea surface salinity (SSS) reconstructions from the subtropical North and South Atlantic combined with general circulation model (GCM) simulations that show synchronous changes in SSS between the hemispheres. Parallel changes in our subtropical SSS records and tropical Caribbean and western Atlantic SSS indicate that a salty surface water mass accumulated in the western Atlantic from 27 degrees South to 33 degrees North during periods of reduced AMOC in agreement with GCM results. However, SSS decreases led AMOC resumption by approximately 500 years, which can be explained by the establishment of shallow overturning circulation that could have triggered through various feedbacks full AMOC resumption. This implies that AMOC may contain an internal, self-limiting feedback.
PP11C-04
Millennial-Scale Increases in Northward AAIW Extent During the Last Deglaciation: Nd Isotope Evidence From the Tropical and Southwest Atlantic
Intermediate water masses are closely involved in the global meridional overturning circulation by compensating for the formation and export of deep waters. The distinct properties of intermediate waters, particularly Antarctic Intermediate Water (AAIW) formed close to the Subantarctic Front in the Southern Ocean, make them important for the redistribution of heat, freshwater and nutrients as well as the density structure of the ocean. AAIW is highly sensitive to current climate warming, suggesting that records of past AAIW variability may hold important clues on ocean-climate connections and the role of AAIW in the global meridional overturning circulation under different climate states. Here we present Nd isotope records from the western tropical Atlantic (MD99-2198: 12°N, 61°W, 1330m water depth) and the southwest Atlantic (KNR159-36GGC: 27.5°S, 46.5°W, 1268m water depth) that indicate marked and abrupt changes in the northward extent of AAIW over the past 25,000 years. Nd isotope ratios increase at both sites at times of Southern Hemisphere warming coincident with the Younger Dryas and Heinrich Event 1 in the North Atlantic, indicating increased northward extent of AAIW at these times. During the last glacial maximum and Antarctic Cold Reversal / Boelling/Alleroed, lower Nd isotope ratios indicate reduced northward extent of AAIW and increased presence of northern component water. These changes are consistent with results from the mid-depth southwest Pacific and are anti-correlated with changes in North Atlantic Deep Water formation, highlighting the important involvement of AAIW variations in rapid reorganizations of the meridional overturning circulation.
PP11C-05
Deglacial circulation changes from dynamical and watermass tracers
We present evidence for deglacial changes in the hydrography and circulation of the North Atlantic ocean. The evidence is based on multiple proxies for rates of flow and changes in the deep and intermediate water mass distributions.
PP11C-06
Pore Fluid Records of the Salinity and Temperature of the LGM Deep Pacific
The [Cl] and δ18O-water profiles of deep-sea pore fluids can constrain the Last Glacial Maximum (LGM) bottom water content of these two tracers. Over a range of sites, and when combined with local measurements of benthic foraminifera, the pore water data yield the T/S plot and the δ18O-S plot for the LGM deep ocean. We report two new pore fluid profiles from the eastern equatorial Pacific that compliment previous work on this topic. IODP site 1239 at 1414 meters water depth yields LGM values for Cl and δ18O that are 4.6±0.6% and 1.2±0.1‰ larger than the modern bottom water. Site 1240 at 3000 meters water depth, on the other hand, has a complex profile for both tracers. This site is in close proximity to the ridge axis and clearly contains enhanced advection of pore fluids, over and above the "normal" model of compaction driven fluid flow. There is evidence for forced flow along the sediment-basalt interface and for enhanced vertical flow due to local temperature gradients. Including both of these sources of advection in our standard 1-D advection-diffusion model results in good fits with the measured [Cl] and δ18O. While the new sources of advection induce large uncertainties in the reconstructed LGM bottom water values, the Holocene portion of the pore fluid profile may be better resolved than at other sites. Both [Cl] and δ18O show a large freshening at 10-20 meters deep in the sediment. This feature may be the result of a global redistribution of ice volume between land and sea during the Holocene that has been recognized, though much more subtly, in our previous profiles.
PP11C-07
North Atlantic Abyssal Circulation During Heinrich Event 1
Pa-231/Th-230 ratios in three sediment cores from the North Atlantic have been used to infer changes in the meridional overturning circulation (MOC) over the past 20 kyr. The large Pa-231/Th-230 ratios (approaching the production ratio of the two radionuclides in the water column) during Heinrich Event 1 (H1) in core OCE326-GGC5 off of the Bermuda rise have been interpreted in terms of a reduced export of Pa-231 to the Southern Ocean owing to a slow-down of the MOC (McManus et al., Nature, 2004). Similar observations have been found in core SU81-18 from the Iberian continental margin (Gherardi et al., EPSL, 2005). The Pa-231/Th-230 record for core DAPC2 from the Norwegian Sea also has elevated values during H1, although variations in the flux of biogenic opal might be an important factor in controlling Pa-231/Th-230 ratios in the sediment (Hall et al., GRL, 2006). Here we will use an inverse method to combine Pa-231/Th-230 observations for H1 in these three cores with a model of the abyssal circulation in the North Atlantic basin. Two null hypotheses will be tested. The first null hypothesis is that the Pa-231/Th-230 data are consistent with the modern circulation. The second is that these data are consistent with a state of no flow in the abyssal region. In testing each hypothesis, due regard will be given to the uncertainties in both the Pa-231/Th-230 data and the model equations. By comparing the adjustments in the Pa-231/Th-230 values that are necessary to bring them into consistency with the modern circulation (hypothesis 1) and the state of rest (hypothesis 2), insight into the dynamical information contained in these data will be gained.
PP11C-08
The Abrupt Climatic Changes During the Last Deglaciation: Direct Land-sea Correlation From a Marine Pollen Record off Southeastern United States
We present a new direct land-sea correlation covering the last deglaciation in order a) to provide a better documentation of the regional vegetation changes in southeastern North America and b) more particularly to assess the connection of the continental climatic changes to North Atlantic circulation rapid variability. It was achieved using coupled analyses of pollen and marine climatic proxies from core KNR140-GGC39 (Blake Outer Ridge) at very high time-resolution. Mg/Ca ratio, planktonic δ18O, mean "sortable silt" grain size (mean S̄S̄) were analyzed in order to get records of SST, salinity and bottom current strength at the core site (Evans et al., submitted to Paleoceanography). The abrupt climatic changes which characterize the last deglaciation, in particular the major cold oscillations Heinrich event 1 (H1) and Younger Dryas (YD), have been widely documented in the North Atlantic and adjacent continents. However, in the tropical and subtropical North Atlantic and southeastern United States, the climatic signature of these events appears quite different and somehow unclear. Our direct land-sea correlation shows three configurations: 1- H1 period: cold climatic conditions in southeastern US (high percentages of boreal and herbaceous taxa) but only extremely cold at around 17 ka, accumulation of salty water in the subtropics (high δ18OSW- IVC) and weak bottom current intensity at the site (low mean S̄S̄) 2- Bolling Alleröd interval: abrupt warming in southeastern US (decrease of boreal taxa in favour of Quercus) at the beginning, synchronous to northern export of the salty water previously accumulated and to an increase of the bottom current strength at the site 3- YD period: mild and wet conditions in southeastern US (expansion of Tsuga and Quercus), decrease of the bottom current strength at the site and accumulation of salty water in the subtropical regions but less than during H1.