Changes In Surface Water Conditions Research Articles

Population dynamics and palaeoenvironmental inferences of Turonian planktonic foraminiferal assemblages from SE Tanzania and ODP Site 762 (Exmouth Plateau, eastern Indian Ocean)

The Tanzania Drilling Program (TDP) recovered hemipelagic sediments from land-based boreholes that yielded extraordinarily well-preserved, diverse Turonian foraminifera. Reliable oxygen and carbon isotopic analyses through most of the Turonian Stage and biostratigraphic data were documented. This study compares Turonian foraminiferal population dynamics and associated geochemical proxy records among the TDP boreholes with correlative records from Ocean Drilling Program Site 762 (Exmouth Plateau, eastern Indian Ocean). The two regions were separated by ∼12° latitude and ∼5000 km of ocean, with the Indian continental plate located between. Taxonomic turnover is similar and is nearly simultaneous in both regions across the transition from the Helvetoglobotruncana helvetica Zone to the Falsotruncana maslakovae Zone (∼93.0 Ma). Changes include overlapping extinctions of all helvetoglobotruncanids, most dicarinellids and one species of Praeglobotruncana and subsequent first appearances of marginotruncanid and falsotruncanid species. Increased abundance of biserial planktonic foraminifera is recorded at multiple TDP sites and at Site 762 across the contact of the Fa. maslakovae and Huberella huberi zones c. 0.5 Myr after the Hv. helvetica / Fa. maslakovae transition. Geochemical records at the two sites do not indicate associated palaeoenvironmental changes in surface water conditions that would explain coordinated changes in species composition on opposites sides of the Indian Ocean.

Surface water mass dynamics at IODP Site U1313 through Principal Component Analysis: Evidence from coccolith assemblages in the ∼25–7 kyr interval

Coccolith assemblages from Integrated Ocean Drilling Program Site U1313 (North Atlantic; 41°0′N, 32°57.4′W) are analysed to assess temporal changes in surface water conditions from the Last Glacial Maximum (LGM) to the Middle Holocene (∼25–7 kyr). We perform Principal Component Analysis (PCA), involving both calcareous nannofossil assemblage data and other paleoenvironmental proxies. We reconstruct sea-surface dynamics using calcareous nannofossil assemblage variations and comparison with other previously published proxy data from Site U1313 including C37:4 alkenone, dolomite/calcite and quartz/calcite ratios, Uk’37 SST, as well as the sedimentation rate.We use variations in paleoproductivity proxies, cold and warm water group abundance, and the PCA results to infer North Atlantic Current (NAC) and North Atlantic Transitional Waters (NATW) migration through the studied interval. These proxies are also used to highlight variations in the Subtropical Gyre dynamics. Warming episodes are recorded during the LGM and Heinrich Stadial (HS) 1 as a result of northward expansion of the Subtropical Gyre. High coccolithophore productivity episodes characterize HS 1 and 2, and are concomitant with increasing terrigenous input. Both processes are considered a consequence of iceberg influence, which transport nutrients and ice-rafted debris to the area. A southward shift of the Subpolar Front and NAC favour the expansion of cold water from 25.3 ka to the end of the Younger Dryas, whereas the early Holocene is characterized by NATW influence resulting from northward migration of the NAC and Azores Current.

Mid-Late Holocene climate variabilities in the Bransfield Strait, Antarctic Peninsula driven by insolation and ENSO activities

The Antarctic Peninsula (AP) is sensitive to global climate changes. Diatom composition was investigated in a sediment core recovered from the Bransfield Strait, in order to reconstruct the changes in surface water conditions over the past ca. 5.8 kyr (mid-late Holocene) in the AP region. A relatively stable mid-Holocene (5.8–3.8 ka) with persistent presence of sea ice was punctuated by a cooling episode at 4.9–4.2 ka, followed by a cooling trend with stronger sea-ice variability during the late Holocene (since 3.8 ka), which enhanced the surface water stratification in the study area. Frequency analysis reveals centennial- to millennial-scale cyclicities resembling the characteristics of solar irradiance and El Niño-Southern Oscillation (ENSO). In particular, the increasing ENSO activities during the late Holocene resulted in intensified cold Weddell Sea outflow to the Bransfield Strait that increased sea ice presence and duration. This pattern is in contrast to a northerly warm air intrusion induced glacial meltwater discharge to the western AP region, leading to similar sea ice variability to that in the east AP region.

Monsoon-induced changes in surface hydrography of the eastern Arabian Sea during the early Pleistocene

AbstractUpper water column dynamics in the eastern Arabian Sea were reconstructed in order to investigate changes in the activity of the South Asian / Indian monsoon during the early Pleistocene (c. 1.5–2.7 Ma). We used planktic foraminiferal assemblage records combined with isotopic (δ18O and δ13C) data, Mg/Ca-based sea surface temperatures and seawater δ18O records to estimate changes in surface water conditions at International Ocean Discovery Program (IODP) Site U1457. Our records indicate two distinct regimes of monsoon-induced changes in upper water structure during the periodsc. 1.55–1.65 Ma andc. 1.85–2.7 Ma. We infer that a more stratified upper water column and oligotrophic mixed layer conditions prevailed during the period 1.85–2.7 Ma, which may be due to overall weaker South Asian / Indian winter (NE) and summer (SW) monsoon circulations. The period 1.55–1.65 Ma was characterized by enhanced eutrophication of the mixed layer, which was probably triggered by intensified winter (NE) monsoonal winds. The long-term trend in hydrographic changes during 1.55–1.65 Ma appears to be superimposed by short-term variations, probably reflecting glacial/interglacial changes. We suggest that an intensification of the South Asian / Indian winter monsoon circulation occurred between ∼1.65 Ma and 1.85 Ma, which is most likely due to the development of strong meridional and zonal atmospheric circulations (i.e. Walker Circulation and Hadley Circulation) because of strong equatorial East–West Pacific temperature gradients.

An analytical framework for quantifying aquifer response time scales associated with transient boundary conditions

Summary A major challenge in studying coupled groundwater and surface-water interactions arises from the considerable difference in the response time scales of groundwater and surface-water systems affected by external forcings. Although coupled models representing the interaction of groundwater and surface-water systems have been studied for over a century, most have focused on groundwater quantity or quality issues rather than response time. In this study, we present an analytical framework, based on the concept of mean action time (MAT), to estimate the time scale required for groundwater systems to respond to changes in surface-water conditions. MAT can be used to estimate the transient response time scale by analyzing the governing mathematical model. This framework does not require any form of transient solution (either numerical or analytical) to the governing equation, yet it provides a closed form mathematical relationship for the response time as a function of the aquifer geometry, boundary conditions, and flow parameters. Our analysis indicates that aquifer systems have three fundamental time scales: (i) a time scale that depends on the intrinsic properties of the aquifer, (ii) a time scale that depends on the intrinsic properties of the boundary condition, and (iii) a time scale that depends on the properties of the entire system. We discuss two practical scenarios where MAT estimates provide useful insights and we test the MAT predictions using new laboratory-scale experimental data sets.

Reconstruction of surface water conditions in the central region of the Okhotsk Sea during the last 180 kyrs

Core GC9A, a 6.7m long gravity core collected from the central region of the Okhotsk Sea during Cruise YK0712 on R/V Yokosuka (JAMSTEC), was used to reconstruct the changes in surface water conditions by measuring biogenic components (biogenic opal, CaCO3, total organic carbon and δ15N of sediment organic matter) of sediment samples. The age of Core GC9A was determined indirectly by graphic correlation comparing the b⁎ (psychometric yellow–blue chromaticness) values with those of well-dated Core MD01-2415, with complement to the tephra layer (K3; 50ka). The bottom age of Core GC9A was estimated to be about 180kyr; therefore it provides the history of surface water conditions from MIS 1 to MIS 6. The biogenic opal, CaCO3, and TOC contents were high during the interglacial periods as expected, indicating enhanced surface water production under warm climatic conditions. This condition resulted from sufficient nutrient supply to the surface waters by active vertical mixing, which was validated by low δ15N values of the sediment organic matter. In contrast, surface water productivity was depressed during the colder glacial periods, probably due to the expanded sea-ice distribution and limited nutrient supply. However, the glacial sediments had moderately high δ15N values, indicating enhanced nitrate utilization resulting from the limited nutrient supply caused by strong stratification of the surface water. High δ15N values were also observed during the deglaciation, which was attributed to the increased nitrate utilization during enhanced surface water productivity. However, the low δ15N values during the glacial and deglacial periods may be attributed to the increased supply of terrestrial organic matter. Diatom production was primarily responsible for surface water paleoproductivity during the interglacial periods rather than coccolithophores. However, the succession of glacial to early deglacial coccolithophore production and late deglacial to interglacial diatom production was remarkable, corresponding to the present-day seasonal phytoplankton succession. Such an advanced coccolithophore production relative to diatom production might be attributed to the degree of nutrient availability associated with surface water conditions on the basis of variations in the δ15N value. Finally, the opal and TOC contents decreased abruptly in conjunction with a gradual decrease in CaCO3 content from about 2ka, which seems to implicate a late Holocene sudden decrease in paleoproductivity in the central region of the Okhotsk Sea. According to the increase in δ15N values during this interval, nutrient availability appears to be poor, which is likely attributed to the resumed strong stratification that occurred due to the southward shift of the Aleutian Low atmospheric pressure system.

Paleoproductivity, ventilation, and organic carbon burial in the Santa Barbara Basin (ODP Site 893, off California) since the last glacial

Biogenic burial rates and coccolith assemblage data were generated for the past 16 ka at ODP Site 893 in the Santa Barbara Basin, off California, to determine if dysoxic events in the basin were related to changes in marine productivity. Coccolith abundance data show that changes in surface water conditions did indeed change in concert with oxygen levels at the sea floor. However, organic carbon burial rates varied independently, indicating that oxygenation of the water column is related mainly to intermediate water ventilation. A strong correlation of organic carbon burial rates with lithogenic, and to a lesser extent with carbonate and opal accumulation rates confirms recent conclusions that the export of organic carbon from the sea surface is largely controlled by the presence of ballast minerals.

Planktonic foraminifera from modern sediments reflect upwelling patterns off Iberia: Insights from a regional transfer function

Quantitative and qualitative analyses of planktonic foraminiferal assemblages from 134 core-top sediment samples collected along the western Iberian margin were used to assess the latitudinal and longitudinal changes in surface water conditions and to calibrate a Sea Surface Temperature (SST) transfer function for this seasonal coastal upwelling region. Q-mode factor analysis performed on relative abundances yielded three factors that explain 96% of the total variance: factor 1 (50%) is exclusively defined by Globigerina bulloides, the most abundant and widespread species, and reflects the modern seasonal (May to September) coastal upwelling areas; factor 2 (32%) is dominated by Neogloboquadrina pachyderma (dextral) and Globorotalia inflata and seems to be associated with the Portugal Current, the descending branch of the North Atlantic Drift; factor 3 (14%) is defined by the tropical–sub-tropical species Globigerinoides ruber (white), Globigerinoides trilobus trilobus, and G. inflata and mirrors the influence of the winter-time eastern branch of the Azores Current. In conjunction with satellite-derived SST for summer and winter seasons integrated over an 18 year period the regional foraminiferal data set is used to calibrate a SST transfer function using Imbrie & Kipp, MAT and SIMMAX ndw techniques. Similar predicted errors (RMSEP), correlation coefficients, and residuals' deviation from SST estimated for both techniques were observed for both seasons. All techniques appear to underestimate SST off the southern Iberia margin, an area mainly occupied by warm waters where upwelling occurs only occasionally, and overestimate SST on the northern part of the west coast of the Iberia margin, where cold waters are present nearly all year round. The comparison of these regional calibrations with former Atlantic and North Atlantic calibrations for two cores, one of which is influenced by upwelling, reveals that the regional one attests more robust paleo-SSTs than for the other approaches.

The carbon isotopic record of the C 37:2 alkenone in the South Atlantic: Last Glacial Maximum (LGM) vs. Holocene

The carbon isotopic fractionation ( ε p) of the C 37:2 alkenone was analysed for 19 South Atlantic sediment samples from the Last Glacial Maximum (LGM). Our study covers the equatorial and subtropical ocean including the coastal upwelling regions off South Africa, the equatorial upwelling, and the oligotrophic western basins. The results were compared to the Holocene ε p records from the respective core locations ( Andersen, N., Müller, P.J., Kirst, G., Schneider, R.R., 1999. Alkenone δ 13C as a proxy for past PCO 2 in surface waters: results from the Late Quaternary Angola Current. In: Fischer, G., Wefer, G. (Eds.), Use of Proxies in Paleoceanography: Examples from the South Atlantic. Springer, Berlin, pp. 469–488; Benthien, A., Andersen, N., Schulte, S., Müller, P.J., Schneider, R.R., Wefer, G., 2002. Carbon isotopic composition of the C 37:2 alkenone in core–top sediments of the South Atlantic Ocean: Effects of CO 2 and nutrient concentrations. Glob. Biogeochem. Cycles 16, 10.1029/2001GB001433 ). Generally, alkenone ε p was lower during the LGM compared to the Holocene. Higher glacial ε p values were only found in sediments from the Angola Basin and in one sample from the eastern crest of the Walvis Ridge. Considering present understanding of LGM–Holocene changes in surface-water conditions (i.e. nutrient level, primary productivity, phytoplankton assemblages), the observed glacial/interglacial difference in ε p indicates that multiple factors controlled the isotopic fractionation in alkenone producing algae depending on the regional setting. In the oligotrophic areas of the South Atlantic the lower than Holocene glacial ε p values can be partly explained with a decrease in surface-water PCO 2 during the LGM. In contrast, the Holocene to LGM decrease in ε p values in the coastal upwelling areas as well as in the eastern tropical Atlantic most probably reflects much higher glacial haptophyte growth rates induced by an increase in surface-water nutrient concentrations. The exceptional opposite trend of the ε p differences in the Angola Basin can be explained by a shift in the phytoplankton community towards a greater dominance of diatoms under glacial conditions, thus leaving less nutrients available for haptophytes. In this way, the isotopic record of alkenones indicate lower haptophyte growth rates during the LGM although other palaeoceanographic proxies point to enhanced productivity and higher nutrient levels.

Surface water conditions in the Northern Benguela Region (SE Atlantic) during the last 450 ky reconstructed from assemblages of planktonic foraminifera

Planktonic foraminiferal records from Site 1083 (ODP Leg 175) were used to investigate changes in surface water conditions in the Northern Benguela Region over the past 450 ky. The assemblages of planktonic foraminifera are dominated by four species: sinistral coiling Neogloboquadrina pachyderma, dextral coiling N. pachyderma, Globigerina bulloides and Globorotalia inflata. Besides, tropical species deliver a small contribution to the assemblage. The most prominent temporal variations, displayed by N. pachyderma (s+d), represent changes in the coastal upwelling and the presence of cold, nutrient rich waters over the core site. Neogloboquadrina pachyderma (s+d) shows cyclic variability in the eccentricity and, to a less extent, precession frequencies. The changes indicate increased upwelling intensity in glacial maxima and precession maxima, and correlate well with the wind-strength record of Stuut et al. (2002). During glacial maxima, steep temperature gradients over the Southern Hemisphere caused strong SE trade winds and strong upwelling. Precession maxima cause a weak monsoonal circulation, more zonal SE trade winds, strong coastal upwelling, and nutrient-rich surface waters over ODP Site 1083. Advection of Angola Current (AC) surface water into the Walvis Basin, indicated by the tropical species, occurs when the Angola Benguela Front (ABF) is positioned southward. Occasionally, this happened during glacial maxima, as can be explained with the reconstructed and predicted meridional movements of the ABF. The amount of AC water was never sufficient to suppress the marine biological production at the core site. The contribution of Benguela Current (BC) water, reflected by Globorotalia inflata, is greatly determined by the upwelling. In periods of strong upwelling, the BC influence is suppressed. In several glacial substages, the temperature of the upwelling South Atlantic Central Water (SACW) may have been increased, as suggested by the dominance of Neogloboquadrina pachyderma (d) in the upwelling record. This phenomenon may be due to intensified subduction in the central South Atlantic that induces the formation of SACW, or to larger contributions of Eastern SACW to the upwelling water. Around 250–200 ky BP, a long-term shift to higher productivity occurred that is absent in the upwelling record. It was accompanied with a transition from a precession and obliquity variability to an eccentricity dominated variability in the Globorotalia inflata (BC) record. The shift was probably connected to a long-term southward shift of the circumpolar oceanic frontal systems south of the African continent.

Late Quaternary Seasonal Sea-Ice History of the Northeastern Japan Sea

The Late Quaternary sea-ice history of the northeastern Japan Sea is discussed on the basis of the occurrence of dropstones and ice-rafted debris (IRD) in fine sediment cores. IRD was found in all strata except those from the Holocene and oxygen isotope stage 5.5. The largest expansion of sea ice was recognized at the last glacial maximum (LGM; oxygen isotope stage 2), when the southern margin of seasonal sea ice was probably located in the vicinity of the Matsumae Plateau. The margin might occasionally have expanded further southward to off the Oga Peninsula. Sea ice expanded southward from mid-stage 5 to the LGM in response to global cooling, but with much fluctuation. Sea ice remained during deglaciation until around 10 ka, but after 10 ka it retreated northward rapidly in response to global warming and changes in surface water conditions. Greater fluctuations in IRD were found in core GH95-1208 collected from off Rumoi, Hokkaido, Japan. More IRD was found in sediments from late stage 3, late stage 5, and early stage 6. The fluctuations were not concordant with global climate changes (based on the standard oxygen isotope curve), and may have been controlled by regional climate factors such as the strength of the winter monsoon, which is related in turn to high-latitude atmospheric circulation.

Sea‐surface conditions in northernmost Baffin Bay during the Holocene: palynological evidence

AbstractThe analysis of cores collected in northernmost Baffin Bay, from within the area of the North Water Polynya, permits definition of a composite sedimentary sequence ca. 12 m thick spanning the last 10 000 14C yr, with only a few discontinuities. Palynological analyses were performed in order to reconstruct changes in surface water conditions and biogenic production. Transfer functions, using dinocyst assemblages, were applied to estimate sea‐surface temperature (SST) and salinity, as well as the seasonal duration of sea ice cover. At the base of the record, prior to 9300 14C yr BP, dinocysts and organic linings of benthic foraminifers are sparse, indicating harsh conditions and low productivity. After ca. 9300 14C yr BP, the increased concentration of benthic foraminifers (up to 103 linings cm−3) and dinocyst fluxes (102–103 cysts cm−2 yr−1) reveals high biological productivity related to open‐water conditions. The early to middle Holocene, from ca. 9000 to ca. 3600 14C yr BP, is marked by relatively high species diversity in dinocyst assemblages and the significant occurrence of autotrophic taxa such as Spiniferites elongatus, Pentapharsodinium dalei and Impagidinium pallidum. This assemblage suggests conditions at least as warm as at present. From ca. 6400 to ca. 3600 14C yr BP, transfer functions indicate warmer conditions than at present, with SST in August fluctuating up to 5.5°C. After 3600 14C yr BP, the dinocyst record suggests a trend of decreasing temperature toward modern values, marked by recurrent cooling events. Copyright © 2001 John Wiley & Sons, Ltd.

Variability in surface and deep water conditions in the nordic seas during the last interglacial period

Stable isotope, foraminifera and ice rafted detritus (IRD) records covering the last interglacial (the Eemian) from 7 sediment cores in a transect from the Norwegian to the Greenland Sea are presented. The percentages of Neogloboquadrina pachyderma (s.) and Globigerina quinqueloba, foraminiferal content, and to some extent planktonic stable isotope records, demonstrate marked, regional changes in surface water conditions. Importantly, the variability in the abundances of subpolar foraminifera and foraminiferal content are not coherent, implying that these two types of proxies fluctuated independently of each other and most likely reflect changes in sea surface temperature and surface water carbonate productivity, respectively. Paleoceanographic reconstructions demonstrate significant movements of the oceanographic fronts. At the warmest periods, the Arctic front was located far west of the present-day location, at least within the Iceland Sea region. At 126–125 ka, this was most probably due to a stronger or more westerly located Norwegian current. Within the later warm intervals, higher heat flux to the western part of the basin reflects a combination of a stronger Irminger current and/or a weaker east Greenland current. During the main cold spell at ∼124 ka, a diffuse Arctic front had a more southeasterly location than today, and intrusion of Atlantic surface waters was probably limited to a narrow corridor in the Eastern Norwegian Sea. A general correspondence between minima in sea surface temperatures and light benthic δ 18O may indicate enhanced influx of freshwater to the basin within the cold events. At least in the Norwegian Sea, we find some evidence that the changes in surface water conditions are associated with changes in deep water ventilation. The majority of the fluctuations may be related to occasional breakdown or reduction of the thermohaline circulation within the Nordic seas. In the earliest Eemian, this could result from meltwater forcing. During the remaining part of the last interglacial the fine balance between temperature and salinity, which the deep water formation is depending on, may have been disturbed by periodic increases in fresh water supply or variable influx of warm Atlantic surface waters.

Evidence for Milankovitch Periodicities in Cenomanian-Turonian Lithologic and Geochemical Cycles, Western Interior U.S.A.

ABSTRACT The limestone/marlstone bedding couplets of the Bridge Creek Limestone Member, Cenomanian-Turonian Greenhorn Formation, were analyzed by applying spectral techniques to high-resolution lithologic and geochemical data from a core. The results suggest that the Bridge Creek contains a complex record of orbital cyclicity. The dominant signal appears to be obliquity, but signals corresponding to precession and eccentricity were also observed. The development of the bedding couplets is interpreted to have resulted from a combination of factors, including insolation-controlled changes in higher-latitude precipitation leading to dilution/redox cycles, and in lower-latitude evaporation, leading to changes in surface water conditions and productivity cycles in the calcareous plankton. The data nterpreted to reflect redox cycles appear to be more strongly influenced by obliquity, and show a weak precessional signal. In contrast, trends in the carbonate record show the opposite response. The complex bedding pattern observed in the Bridge Creek Limestone is interpreted to result from the competing influences of different orbital cycles expressed through different pathways of the depositional system, and was also affected by changes in sedimentation rates related to relative sea level fluctuations, aperiodic dilution by volcanic ash, and changes in organic-matter production and redox conditions related to a global oceanic anoxic event. These factors complicate cycle analysis in the lower part of the member but leave a relatively undisturbed record in the upper Bridge Creek Limes one.