Changes In Surface Salinity Research Articles

The impact of a severe drought on the vegetation of a subtropical estuary

In order to document the effect of the recent drought and the resulting marine intrusion event on plant-community shifts in a Louisiana estuary, we analyzed two vegetation data sets collected in Barataria estuary in 1997 and 2000 and compared community shifts to surface salinity changes at four points along the estuarine gradient within the study area. We used the major vegetation types identified in our previous research of larger data sets and tested the use of a simple vegetation classification technique. This vegetation classification technique is based primarily on the dominant and co-dominant species, and secondarily on the number of taxa observed. To distinguish vegetation types with similar dominant species but different associated species, the vegetation classification technique used a salinity score derived from the species composition. Surface water salinity increases were reflected by a change in species composition in the mesohaline to fresh marshes. The largest species composition shift observed was the shift from oligohaline wiregrass (species rich vegetation type dominated bySpartina patens) to mesohaline wiregrass (vegetation type dominated byS. patens with few other species). Shifts in vegetation composition may have been enhanced by the presence of the major dominant species at a low abundance in other vegetation types. The vegetation classification technique used could classify over 95% of the stations. This vegetation classification technique provides a simple method to classify Louisiana's coastal vegetation based on plant species composition.

Sea surface salinity changes in the East China Sea during 1997–2001: Influence of the Yangtze River

The role of the Yangtze River discharge in changing sea‐surface salinity (SSS) in the East China Sea during the 1997–2001 periods is investigated by using (1) in situ SSS measurements along a merchant ship track running from Taiwan strait to Korea strait, (2) ocean general circulation model (OGCM) simulations and specific sensitivity studies, and (3) in land precipitation data to infer estimates of the river discharge in absence of gauged station data. The river effects on 1997–2001 sea surface temperature (SST) and surface currents are also examined. Both in situ SSS‐SST measurements and OGCM simulations clearly indicate that the integration of the 1997 drought signal and 1998–1999 flooding signal over the catchment area of the Yangtze River are manifest in the ocean where local precipitation played a rather modest role. Sensitivity studies with the OGCM with and without the river discharge further reveal that these signals can extend over several hundred kilometers off the river mouth, consistent with the along track in situ measurements. In particular, SSS along the ship track can drop from about 34 psu in August 1997 to less than 26 psu in July 1998 (almost the same for 1999) at the latitude of the river mouth although located 500 km to the west of the ship track. Model results indicate that the dynamic and thermodynamic effects of the freshwater input from the river may be phase lagged by up to a season with maximum surface warming occurring in the summer months when the river discharge is maximum whereas the largest differences in mixed layers with and without the river input occur during later winter and late autumn. The possible positive feedback between the increase of river runoff yielding warm SST anomaly off the river mouth and so enhanced atmospheric convection and precipitation is discussed.

Dinoflagellate cysts, freshwater algae and fungal spores as salinity indicators in Late Quaternary cores from Marmara and Black seas

Seismic profiles and mollusks have been used to suggest that from ∼12 500 to 7000 yr BP, the Black Sea was an isolated freshwater lake containing potable water and implying a surface salinity of <1. According to Ryan and Pitman (1999), these circumstances encouraged Neolithic settlement and farming on the shore of the Black Sea. This model conflicts with previous studies of dinoflagellate cysts and seismic profiles from the Marmara Sea. Here we investigate Ryan and Pittman’s model using palynological studies of organic-walled dinoflagellate cysts, acritarchs, freshwater algae, microforaminifera, and fungal remains as tracers of changes in surface salinity for seven cores of pleniglacial to Holocene sediments from the Marmara and Black seas. Core-top data from 16 sites along a salinity gradient from 39.5 in the Aegean Sea to ∼17 in the Black Sea show that the dinocyst species Impagidinium aculeatum, Impagidinium patulum, Operculodinium israelianum, Polysphaeridium zoharyi and Nematosphaeropsis labyrinthus are markers of Mediterranean Sea water and summer surface salinity >24, and that Spiniferites cruciformis, Spiniferites inaequalis, Peridinium ponticum, Polykrikos spp. and Quinquecuspis concreta characterize the lower salinity of the Marmara and/or Black seas. The core-top data and correlatable down-core assemblage changes in time-equivalent sapropelic and brown muds show that there is no evidence for differential aerobic decay of dinocysts in the study area. The main acritarchs are Sigmopollis psilatum, Concentricystes cf. C. rubinus and cf. Acritarch-8 of Traverse (1978), all of which are absent from the Aegean Sea and decrease in abundance with increasing salinity; the first two taxa have been reported previously as freshwater species. Fungal remains show a similar distribution pattern to the freshwater acritarchs, indicating their origin from terrestrial environments. Freshwater Chlorococcales are almost confined to the Black Sea but they have rare occurrences in the Aegean, indicating long-distance transport. Microforaminiferal linings are abundant in the Marmara Sea but are absent in deep water of the Black Sea. In the Marmara Sea, mid–late Holocene assemblages (<7 ka) are dominated by Lingulodinium machaerophorum, Operculodinium centrocarpum and halophilic Mediterranean Spiniferites spp. ( S. mirabilis, S. hyperacanthus, S. bentorii), and a diversity of heterotrophic protoperidinioid and Polykrikos species are present in both sapropels and brown lutites. In contrast, the early Holocene interval (including sapropels) has fewer halophilic Spiniferites spp. and other Mediterranean taxa ( O. israelianum, Polysphaeridium zoharyi) and more low salinity indicators ( P. ponticum, Pyxidinopsis psilata and S. cruciformis), suggesting that the overflow of Black Sea water was greater than now. The time-equivalent early Holocene unit in the Black Sea has a higher percentage of taxa found in freshwater environments (including S. cruciformis forms 1–3; Gonyaulax apiculata, fungal remains and freshwater acritarchs) but the continued presence of Spiniferites morphotypes and other taxa associated with brackish (∼7–18) to saline surface water indicates that there was some Mediterranean water present at this time. The late glacial and Pleniglacial sediments in the Marmara Sea contain a lower diversity of dinoflagellates, dominated by S. cruciformis and P. psilata, as in early Holocene sediments of the Black Sea but including the short-spine morphotypes of S. cruciformis which are not found in freshwater lakes. We conclude that although the glacial stages were marked by much lower sea-surface temperatures, there was either some periodic marine influence or the marine dinocysts were living in a brackish water environment (∼7–18) with the freshwater species being transported from glacial lakes. Overall, there is no palynological evidence that the surface salinity of the Black or Marmara seas was ever as low as a freshwater lake. This finding is consistent with models that estimate the time required to desalinate the Black Sea after closing of the Marmara–Bosphorus gateway.

Interannual sea surface salinity and temperature changes in the western Pacific warm pool during 1992–2000

Sea surface salinity (SSS) and sea surface temperature (SST) in the western Pacific warm pool (130–180°E; 10°N–10°S) are analyzed for the period 1992–2000 taking advantage of complementary data from the ship of opportunity program and the Tropical Atmosphere‐Ocean (TAO)‐Triangle Trans‐Ocean Buoy Network (TRITON) array of moored buoys. Covariability of these variables with surface wind stress, surface zonal currents, evaporation, precipitation, and barrier layer thickness is also examined. These fields all go through large oscillations related to the El Niño Southern Oscillation (ENSO) cycle, most notably during the record breaking 1997–1998 El Niño and subsequent strong 1998–2000 La Niña. East of about 160°E, during El Niño, precipitation minus evaporation increases in the equatorial band, in conjunction with anomalous increases in westerly winds, eastward surface currents, SST, and decreases in SSS. Opposite tendencies are evident during La Niña. Peak to peak 2°N–2°S averaged variations reached as much as 1.2 m s−1 for zonal currents and 1.5 practical salinity units (psu) for SSS. West of about 160°E, SST cools during El Niño and warms during La Niña, opposite to what occurs further east. To understand these SST tendencies west of 160°E, a proxy indicator for barrier layer formation is developed in terms of changes in the zonal gradient of SSS (∂S/∂x). Zonal SSS gradients have been shown in modeling studies to be related to barrier layer formation via subduction driven by converging zonal currents in the vicinity of the salinity front at the eastern edge of the warm pool. Correlation between changes in ∂S/∂x and changes in SST a few degrees longitude to the west is significantly nonzero, consistent with the idea that increased barrier layer thickness is related to warmer SSTs during periods of westward surface flow associated with La Niña, and vice versa during El Niño. Direct evidence of barrier layer thickness variations in support of this hypothesis is also presented.

Meltwater and the global ocean conveyor: northern versus southern connections

The sensitivity of the ocean circulation to changes in North Atlantic surface fluxes has become a major factor in explaining climate variability. The role of the Antarctic Bottom Water in modulating this variability has received much less attention, limiting the development of a complete understanding of decadal to millennial time-scale climate change. New analyses indicate that the southern deepwater source may change dramatically (e.g., experience a decrease of as much as two thirds during last 800 years). Such change can substantially alter the ocean circulation patterns of the last millennium. Additional analyses indicate that the Southern Hemisphere led the Northern Hemisphere changes in some of the glacial cycles of Pleistocene, implying a seesaw-type oscillation of the global ocean conveyor. The potential for melting of sea ice and ice sheets in the Antarctica associated with global warming can cause a further slowdown of the southern deepwater source. These results demand an assessment of the role of the Southern Ocean in driving changes of the global ocean circulation and climate. Systematic model simulation targeting the ocean circulation response to changes in surface salinity in the high latitudes of both Northern and Southern Hemispheres demonstrate that meltwater impacts in one hemisphere may lead to a strengthening of the thermohaline conveyor driven by the source in the opposite hemisphere. This, in turn, leads to significant changes in poleward heat transport. Further, meltwater events can lead to deep-sea warming and thermal expansion of abyssal water, that in turn cause a substantial sea-level change even without a major ice sheet melting.

Seasonal and Interannual Variability in a Model of the Mediterranean under Derived Flux Forcing

Abstract A 100-yr integration of a Mediterranean Sea model is performed with surface restoring to monthly varying T, S, followed by 100 years of integration with surface fluxes of heat and freshwater alone. The fluxes are diagnosed from the restoring boundary run and are shown to agree favorably with the observations. The model properties remain stable under flux forcing, reproducing all major water masses and a realistic thermohaline circulation, although with more variability than the restoring run. Annual average production of Levantine Intermediate Water (LIW) is 1.3 ± 0.3 Sv on a core density of σθ = 29.05. A major component of variation is on a timescale of 2–3 yr and is correlated with changes in surface salinity in the Levantine by 0.1 psu, which are in turn related to a variable path for the modified Atlantic water in the eastern basin. Smaller variations in production of deep waters in the Adriatic and Western Mediterranean are also found to be significantly correlated with the LIW production at...

Interannual Variability of the Subsurface High Salinity Tongue South of the Equator at 165°E*

A 14-yr time series of salinity at thermocline level was constructed from repeated meridional CTD sections (averaging about 110 days apart) spanning the equator along 165°E during 1984–97. A tongue of high salinity water extends along the isopycnal σt = 24.5 from its surface outcrop in the southeast Pacific to 175-m depth near 5°–10°S along the section at 165°E. In the west, the tongue moves vertically with the thermocline, mostly as part of the ENSO cycle, while salinity in the tongue varied interannually over a range of 0.4 psu. Most of this variability was due to zonal advection along the isopycnal tongue, with similar changes observed at other longitudes in the west and central Pacific. Part of the interannual salinity signal can be attributed to changes in the near-zonal flow of the South Equatorial Current associated with El Niño, but a general 0.3 psu rise occurring during the 1990s (probably starting even earlier) was not apparently consistent with this explanation. An attempt was made to trace the source of these changes to surface fluxes at the outcrop, where the variation of evaporation and precipitation suggested salinity anomalies of the same magnitude as the subsurface changes. However, the implied surface salinity changes were of the wrong sign to explain the subsequent downstream subsurface variability, and therefore present observations do not demonstrate any influence of subduction of surface properties on salinity in the southern high salinity tongue.

Sources of sediment to the Ionian Sea and western Levantine basin of the Eastern Mediterranean during S-1 sapropel times

The provenance of sediments to the Ionian Sea and western Levantine basin during S-1 sapropel times was studied using 87 Sr/ 86 Sr isotopic ratios together with major elements. Measurements were made on sediment profiles from 5 box cores in the Eastern Mediterranean, on Saharan dust and on a core from the Nile delta which included sediment deposited during S-1 sapropel times. In the sediment profiles, there was a decrease in 87 Sr/ 86 Sr ratio, 1/Sr, Ti/Al, Mg/Al and Si/Al ratio starting at the upper Mn peak and finishing at the base of the organic-rich layer. This showed that there was a major change in the nature of the detrital input during S-1 sapropel times. The timing of the change in detrital input was similar to the change in surface salinity as measured by δ 18 O . There was linear relationship between 87 Sr/ 86 Sr and 1/Sr and for 87 Sr/ 86 Sr and Ti/Al for all the sediment samples. The favoured interpretation of this pattern was that the detrital input was a mixture of Saharan dust, similar to that being deposited today on Corsica, and a local riverine sediment source. The data were incompatible with the riverine source to the Ionian Sea and western Levantine basin, being from the river Nile. The minor variations in Sr isotopes, and metal/Al ratios in the Nile delta core was interpreted as due to changes in the proportion of Blue Nile to White Nile inputs brought about by climatic variations. It was calculated that the change in detrital input during S-1 time was caused by a simultaneous increase in riverine input and decrease in Saharan dust input.

The Southern Ocean response to global warming in the CSIRO coupled ocean-atmosphere model

The Southern Ocean response to global warming is examined for a transient greenhouse gas integration using the Commonwealth Scientific and Industrial Research Organisation (CSIRO) coupled ocean-atmosphere model. The ocean component includes the Gent and McWilliams scheme for adiabatic eddy-induced transport. The atmospheric equivalent CO 2 concentration (CO e 2) follows the IPCC/IS92a radiative forcing scenario from 1880 to 2082, and is then maintained at a constant value of three times the 1880 level for seven centuries. The simulated changes which occur in the Southern Ocean under global warming are very profound, and they begin to separate clearly from the background climate noise in the model's 1990s. A major reduction in the depth and extent of convective mixing occurs by the time of CO e 2 doubling (year 2033), with near-cessation of convection by the time of CO e 2 tripling. Similarly major reduction occurs in the downwelling adjacent to Antarctica associated with Antarctic Bottom Water formation, which also nearly ceases by the time of CO e 2 tripling. These changes are associated with a marked reduction in surface density and salinity. By the time of CO e 2 tripling, both the pycnocline and halocline south from 60°S intensify over the control by about a factor of four. The changes in surface salinity and density continue to intensify for several centuries during the subsequent period of elevated stable CO e 2, and convection and Antarctic downwelling do not recover at all for the duration of the transient run. The water of the entire global ocean below about 1.5 km depth remains stagnant for the duration of the period of elevated stable CO e 2, retaining a density which is too great to allow renewal from any source. Possible caveats on the realism of these results are discussed, and potential consequences of the above changes are noted.

Air–Sea Feedback in the North Atlantic and Surface Boundary Conditions for Ocean Models

Extratropical sea surface temperature (SST) and surface turbulent heat flux monthly anomalies in the central and eastern part of the North Atlantic are considered for the period 1952–92 on a 5° × 5° grid. In this region where the mean surface current is small, the SST anomalies are well simulated by a simple one-dimensional mixed layer model that is stochastically forced by the day-to-day changes in the local air–sea fluxes. A statistical signature of the stochastic model is that the cross correlation between surface heat flux and SST anomalies changes sign between negative and positive lags when the heat flux feedback is negative. This is observed at each grid point of the domain for the turbulent heat flux, which thus contributes both to generating the midlatitude SST anomalies and to damping them, once they are generated. Using properties of the lag covariance between SST and heat flux anomalies, the turbulent heat flux feedback is estimated from the observations. It averages to about 20 W m−2 K−1 in the investigated domain, increasing toward the northwest and the northeast and decreasing southward. It also varies seasonally, being generally largest in the fall and smaller and more uniform in summer. There is no indication that it can become significantly positive. A negative turbulent heat flux feedback is also suggested by the lag relation between the dominant modes of SST and turbulent heat flux variability over the whole North Atlantic, and it is found that the spatial patterns of the associated SST and turbulent heat flux anomalies are remarkably similar whether the atmosphere leads or lags, with only a change of heat flux sign between lead and lag situations. This analysis provides some observational support for the use on short timescales of a restoring condition for SST in ocean-only simulations, but the coupling coefficient should be weaker than usually assumed and a function of latitude and season. The associated SST–evaporation feedback has little effect on interannual surface salinity changes. It should be significant on longer timescales, but then the restoring temperature should be allowed to vary and nonlocal influences should be considered.

Sea surface salinity changes along the Fiji‐Japan shipping track during the 1996 La Niña and 1997 El Niño period

Sea‐surface salinity (SSS) changes during the 1996 La Niña and 1997 El Niño events are analysed along the Fiji‐Japan shipping track, based on 20 thermosalinograph sections. In the equatorial band, above‐average SSS (35.2 to 35.4 instead of 35) were observed in 1996, consistent with a well‐marked south equatorial current, an unusually‐strong equatorial upwelling, and below‐average precipitation (P). From January to August 1997, the SSS decreased sharply from 35.2 to 33.8 (lowest recorded monthly value over the last 20 years), compatible with a reversal of zonal current, the occurrence of equatorial downwelling, and above‐average P. From September to November 1997, the SSS remained almost constant (34.2), consistent with the opposite effects of eastward current, likely bringing low saline water from the Pacific warm pool, and of evaporative cooling, vertical mixing and below‐average P which all tend to increase SSS. The potential impacts of the observed SSS changes on sea level are discussed.

Net zooplankton and the biological pump: a comparison between the oligotrophic and mesotrophic equatorial Pacific

Structure and functioning of the zooplankton community and their consequences on the export fluxes of carbon, nitrogen and phosphorus were studied in two contrasting systems of the equatorial Pacific: the oligotrophic TTS (Typical Tropical Structure) to the west, and the mesotrophic HNLC (High Nutrient-Low Chlorophyll) in the central Pacific. Data were collected during the FLUPAC cruise equatorial transect (September-October 1994) of R.V. L'Atalante and four 6–8-day long time-series stations made between 165°E and 150°W. Along the equator, a sharp 2.5-fold increase in mesozooplankton biomass (200–2000 pm) was observed between 174° and 172°W, with a simultaneous change in surface salinity and chlorophyll concentration, corresponding to the shift between the TTS and HNLC systems. No significant zonal trend was observed within the two systems. Compared with TTS, HNLC presented a significantly greater contribution of the (500–2000 μm) size class to total mesozooplankton biomass, less diel variations, and a shallower vertical distribution. Lower metabolic rates in HNLC were accounted for by different taxonomic compositions in the two areas. Microzooplankton (35–200 gm) had a rather uniform biomass in TTS and HNLC, presented no significant diel variations in the 0–100 or 0–200 m layers, and displayed a shallower vertical distribution than the mesozooplankton. Consequences of such zooplanktonic features on the “biological pump” are assessed. Dissolved nitrogen (DN) and phosphorus (DP) active fluxes, resulting from excretion of interzonal mesozooplankton migrants were 2.1 times higher in the TTS than in the HNLC. However, dissolved inorganic carbon active fluxes were equal in the TTS and HNLC systems, due to differences in C, N, P metabolisms. Combined mesozooplankton and estimated micronekton nitrogen active fluxes represented 40% of the passive flux as measured by sediment traps in the TTS, and 9% in the HNLC. Estimates of mesozooplankton fecal production in the pbotic zone lead to a 2-fold increment between the oligotrophic and mesotrophic stations, and a larger contribution of the fecal production to the sinking flux. It is, therefore, concluded that the mesozooplankton role in the biological pump is mainly passive in the HNLC system, in contrast to the TTS site.

The Long-Term Hydrographic Record at Bermuda

Abstract A long, by oceanographic standards, time series of hydrographic observations at Bermuda was begun in 1954 and continues to the present. Analysis of this dataset has shown the temperature and salinity variations on interannual timescales to be largely independent in the surface layer (0–200 dbar), where integral timescales for salinity are 50% longer than for temperature and surface salinity changes are correlated with decadal changes in the 18°C Water production. Temperature/salinity anomalies are highly correlated in the thermocline where the interannual variability at that level is accountable to either vertical oscillations of the thermocline with amplitudes of ±50 m or meridional oscillations of the horizontal gradient set up at the southern edge of the recirculation gyre of ±300 kim. Salinity changes in the deepest layer observed at Bermuda station “S” (1500–2500 dbar) are uncorrelated with temperature, masked by measurement errors in the early years of the time series. Inclusion of earlier ...

High resolution stratigraphy of the Faeroe-Shetland Channel and its relation to North Atlantic paleoceanography: the last 87 kyr

Two piston cores from the Faeroe-Shetland Channel have been investigated for stratigraphic and paleoceanographic correlation with Atlantic Ocean and Norwegian Sea records. The high resolution cores have been analyzed for the downcore distribution of planktic and benthic foraminifera and oxygen isotopes. AMS 14C dates and the position of Ash layers I and II form the basis for the correlation. Core NA81-10 covers the period from isotope stage 3 to Recent (stage 1), while core NA81-04 covers isotope stages 5a to 1. The Holocene is well represented in core NA81-10 by 2 m of sediment, whereas it is only 50 cm in core NA81-04 which is situated directly under the main core of the Norwegian Sea Deep Water outflow. The transition of isotope stage 2 to stage 1 is affected by dissolution and thus is less clearly represented. AMS 14C dates show that Termination IA begins at 16 ka and ends at about the 13 ka level. The Last Glacial Maximum (LGM; defined at 19-16 ka) is characterized by an IRD peak, faunal (planktic foraminifera) maxima (dated at 17.3 and 18.25 ka) and a high percentage of Globigerina quinqueloba. Apparently open water conditions and circulation within the Norwegian Sea prevailed at least from 22.5 to 14.8 ka, during the greatest part of stage 2 (including the LGM) and well onto Termination IA. A preservation maximum is observed throughout stage 2. Isotope stage 3, with a nearly complete record of isotope substages is well represented in both cores. Oxygen isotope depletion peaks correlate with fauna minima and in time to North Atlantic Heinrich events (H5-H1). Faunal data indicate that deep water is generated within the Norwegian-Greenland Sea repeatedly since 26 ka, with stops prior to 26 ka, 22.5–20.8/20.5 ka and between 14.9 and 13 ka (H3, H2 and H1). All isotope depletion peaks since ca. 50 ka are anticorrelated with fauna and are mainly controlled by surface salinity changes. Isotope stages and substages are directly correlated to fauna peaks and temperature between 87 and ca. 50 ka. The Faeroe-Shetland Channel record shows that repeated presences of Atlantic Surface Water and deep convection alternating with convection shutdowns have occurred within the Norwegian Sea-Greenland Sea from the beginning of stages 5a to 1.

Hydrographic changes of the Southern Ocean (southeast Indian Sector) Over the last 230 kyr

Hydrographical changes of the southern Indian Ocean over the last 230 kyr, is reconstructed using a 17‐m‐long sediment core (MD 88 770; 46°01′S 96°28′E, 3290m). The oxygen and carbon isotopic composition of planktonic (N. pachyderma sinistra and G. bulloides) and benthic (Cibicidoides wuellerstorfi, Epistominella exigua, and Melonis barleeanum) foraminifera have been analysed. Changes in sea surface temperatures (SST) are calculated using diatom and foraminiferal transfer functions. A new core top calibration for the Southern Ocean allows an extension of the method developed in the North Atlantic to estimate paleosalinities (Duplessy et al., 1991). The age scale is built using accelerator mass spectrometry (AMS) 14C dating of N. pachyderma s. for the last 35 kyr, and an astronomical age scale beyond. Changes in surface temperature and salinity clearly lead (by 3 to 7 kyr) deep water variations. Thus changes in deep water circulation are not the cause of the early response of the surface Southern Ocean to climatic changes. We suggest that the early warming and cooling of the Southern Ocean result from at least two processes acting in different orbital bands and latitudes: (1) seasonality modulated by obliquity affects the high‐latitude ocean surface albedo (sea ice coverage) and heat transfer to and from the atmosphere; (2) low‐latitude insolation modulated by precession influences directly the atmosphere dynamic and related precipitation/ evaporation changes, which may significantly change heat transfer to the high southern latitudes, through their control on latitudinal distribution of the major frontal zones and on the conditions of intermediate and deep water formation.

Surface water temperature, salinity, and density changes in the northeast Atlantic during the last 45,000 years: Heinrich events, deep water formation, and climatic rebounds

We developed a new method to calculate sea surface salinities (SSS) and densities (SSD) from planktonic foraminiferal δ18O and sea surface temperatures (SST) as determined from planktonic foraminiferal species abundances. SST, SSS, and SSD records were calculated for the last 45,000 years for Biogeochemical Oceanic Flux Study (BOFS) cores 5K and 8K recovered from the northeast Atlantic. The strongest feature is the dramatic drop in all three parameters during the Heinrich “ice‐rafting” events. We modelled the possibility of deepwater formation in die northeast Atlantic from the SSD records, by assuming that the surface waters at our sites cooled as they flowed further north. Comparison with modelled North Atlantic deepwater densities indicates that there could have been periods of deepwater formation between 45,000 and 30,000 14C years B.P. (interrupted by iceberg meltwater input of Heinrich event 3 and 4, at 27,000 and 38,000 14C years B.P.) and during the Holocene. No amount of cooling in the northeast Atlantic between 30,000 and 13,000 years could cause deep water to form, because of the low salinities resulting from the high meltwater inputs from icebergs. Our records indicate that after each Heinrich event there were periods of climatic rebound, with milder conditions persisting for up to 2000 years, as indicated by the presence of warmer and more saline water masses. After these warm periods conditions returned to average glacial levels. These short term cold and warm episodes in the northeast Atlantic are superimposed on the general trend towards colder conditions of the Last Glacial Maximum (LGM). Heinrich event 1 appears to be unique as it occurs as insolation rose and was coeval with the initial melting of the Fennoscandian ice sheet. We propose that meltwater input of Heinrich event 1 significantly reduced North Atlantic Deep Water formation, reducing the heat exchange between the low and high latitudes, thus delaying deglaciation by about 1500 radiocarbon years (2000 calendar years).

A decadal oscillation due to the coupling between an ocean circulation model and a thermodynamic sea-ice model

A 3-dimensional, planetary-geostrophic, ocean general circulation model is coupled to a thermodynamic sea-ice model. The thermal coupling takes account of the insulating effect of the ice. A simple approach is taken in the case of the freshwater flux by allowing this to pass through the ice, except that some is used for snow accumulation. It is then modified by salinity rejection/dilution due to freezing/melting. The model has idealized box geometry extending 60° in both latitude and longitude, with a horizontal resolution of 2° and 14 vertical levels. Annual mean surface forcings are used. The coupled system is first spun up using restoring conditions on both surface temperature and surface salinity to reach a steady state which includes ice in the high latitudes. A switch of the surface forcing to mixed boundary conditions (restoring on temperature and flux on salinity) leads to an oscillation of period 17 years in the magnitude of the thermohaline circulation and the ice extent. The oscillation is due to a feedback between ice cover and ocean temperature. Since ice forms only in regions where the ocean loses heat to the atmosphere, the thermal insulation of an increased ice cover makes the ocean warmer. The thermohaline circulation plays a role in transporting this heat polewards, which in turn melts the ice. The heat loss over open water at high latitudes then leads to ice formation and the process repeats itself. Salinity rejection/dilution associated with ice formation/melting is shown to be of secondary importance in this oscillation. Rather, changes in surface salinity are dominated by changes in deep convection and the associated vertical mixing, which are themselves associated with the reduction in surface heat loss due to the insulating effect of the ice. As a consequence the model exhibits the negative correlation between surface salinity and ice extent that is observed in the high latitude North Atlantic

A shell‐derived time history of bomb 14C on Georges Bank and its Labrador Sea implications

Bomb‐produced radiocarbon has been used in the past as an important tracer of ocean circulation and as a valuable tool for calculating CO2 air‐sea exchange. However, previous studies of the ocean's time‐varying bomb 14C record have been confined exclusively to analyzing banded corals, and thus their application has been limited to the lower latitudes. The first time history of bomb 14C from the high‐latitude North Atlantic Ocean is obtained from a 54‐year‐old mollusc specimen, (Bivalvia) Arctica islandica, which was collected live from Georges Bank (41°N) in 1990. The annual growth bands of its shell were analyzed for Δ14C using accelerator mass spectrometry, producing a Δ14C time history from 1939 to 1990. The depleted condition of the Georges Bank bomb 14C signal relative to two coral‐derived North Atlantic Δ14C time histories suggests a significant deepwater source for the waters on Georges Bank. Supported by previous work linking the origin of waters on Georges Bank to the Labrador Sea, the Δ14C budget on Georges Bank is modeled as Labrador Sea water, which largely becomes confined to the shelf and partially equilibrates with the atmosphere during a 1‐year transit time from the Labrador Sea to Georges Bank. This model is also used to estimate a time history of bomb 14C for the Labrador Sea. Prebomb Δ14C values calculated for the surface Labrador Sea suggest that a greater inventory of bomb 14C has accumulated here than has previously been reported. However, the estimated prebomb average Δ14C (−70.6‰) for this period is nearly identical to the −70‰ previously calculated for the prebomb source of North Atlantic Deep Water and is in agreement with Transient Tracers in the Ocean subsurface tritium data from the central Labrador Sea. Deduced variations in the ventilation and/or 14CO2 uptake rates in the Labrador Sea correspond with observed changes in surface salinity of the Labrador Sea, suggesting a reduction in deepwater formation during the late 1960s and 1970s.

Changes in surface salinity of the North Atlantic Ocean during the last deglaciation

ABRUPT and short climate changes, such as the Younger Dryas, punctuated the last glacial-to-interglacial transition1–4. Broecker et al.5 proposed that these may have been caused by an interruption of thermohaline circulation as inputs of glacial meltwater freshened the surface waters of the North Atlantic. The finding6that meltwater discharge was minimal during the Younger Dryas, however, led to the suggestion that the surface-water salinity drop might have been caused instead by changes in the freshwater budget (the difference between precipitation and evaporation), accompanied by a reduction in poleward advection of saline subtropical water. Here we use micropalaeontological and stable-isotope records from foraminifera in two cores from the North Atlantic to generate two continuous, high-resolution records of sea surface temperature and salinity changes over the past 18,000 years. Despite the injection of glacial meltwater during warm episodes, we find that sea surface salinity and temperature remain positively correlated during deglaciation. Cold, low-salinity events occurred during the early stages of deglaciation (14,500–13,000 years ago) and the Younger Dryas, but the minor injections of meltwater at high latitudes during these events are insufficient to account for the observed salinity changes. We conclude that an additional feedback from changes in the hydrological cycle and in advection was necessary to trigger changes in thermohaline circulation and thus in climate. This feedback did not act when the meltwater injection occurred at low latitude.

The record of deglaciation in the Sulu Sea: Evidence for the Younger Dryas Event in the tropical western Pacific

A high‐resolution, accelerator mass spectroscopy 14C dated sediment record from the Sulu Sea clearly indicates that the Younger Dryas event affected the western equatorial Pacific. Planktonic foraminiferal δ18O and abundance data both record significant changes during Younger Dryas time. In particular, a 0.4‰ increase in the δ18O value of Globigerinoides ruber and the reappearance of the cool water planktonic foraminifera, Neogloboquadrina pachyderma, occur during the Younger Dryas at this location. These isotopic and faunal changes are a response to either surface water temperature or salinity changes, or some combination of the two. Changes in surface salinities could have been accomplished through either local or global processes. Intensification of the monsoon climate system and increased precipitation at approximately 11 ka is one mechanism that may have resulted in local changes in salinity. A meltwater pulse derived from the Tibetan Plateau is another mechanism which may have caused local changes in salinity. The presence of the Younger Dryas in the tropical western Pacific clearly indicates that this climatic event is not restricted to the North Atlantic or high latitudes, but rather is global in extent.