Monika Segl

Trade wind forcing of upwelling, seasonally, and Heinrich events as a response to sub‐Milankovitch climate variability

Planktonic foraminifera recovered from two cores in the northern Benguela upwelling system reveal a history of rapid events with a variability at sub-Milankovitch frequencies during the last 140 kyr. The “cold-water” planktonic foraminifer, left coiling Neogloboquadrina pachyderma (Ehrenberg), shows rapid fluctuations in relative abundance, indicating changes in upwelling intensity. The periods of high abundance in left coiling N. pachyderma are referred to as “PS events” (pachyderma sinistral) and indicate increased intensity and zonality of the South Atlantic trade winds controlling the Benguela upwelling system. The good correlation between PS events, the North Atlantic Heinrich events, and the Dansgaard-Oeschger cycles from the Greenland Ice Sheet Program (GISP2) ice core suggests large-scale global oceanographic or climatic teleconnections between the South and North Atlantic via the trade wind system. The radiocarbon constrained timing of PS events younger than 45 kyr indicates that the South Atlantic leads the North Atlantics response to trade wind changes, particularly during isotope stages 4–2 when the Earth was dominated by large ice sheets. At times of increased trade wind strength, tropical and subtropical waters are forced across the equator enhancing the pool of warm water to be transferred to the high latitudes of the North Atlantic via the Gulf Stream and North Atlantic Drift, increasing the pull of the thermohaline convective conveyor. The increased supply of warm water to the polar regions of the northern hemisphere increases the ice-ocean moisture gradient and accelerates ice sheet growth, leading to eventual instability and collapse.

Late Quaternary Surface Circulation of the South Atlantic: The Stable Isotope Record and Implications for Heat Transport and Productivity

The central problem of late Quaternary circulation in the South Atlantic is its role in transfer of heat to the North Atlantic, as this modifies amplitude, and perhaps phase, of glacial- interglacial fluctuations. Here we attempt to define the problem and establish ways to attack it. We identify several crucial elements in the dynamics of heat export: (1) warm-water pile-up (and lack thereof) in the western equatorial Atlantic, (2) general spin-up (or spin-down) of central gyre, tied to SE trades, (3) opening and closing of Cape Valve (Agulhas retroflection), (4) deepwater E-W asymmetry. Means for reconstruction are biogeography, stable isotopes, and productivity proxies. Main results concern overall glacial-interglacial contrast (less pile-up, more spin-up, Cape Valve closed, less NADW during glacial time), dominance of precessional signal in tropics, phase shifts in precessional response. To generate working hypotheses about the dynamics of surface water circulation in the South Atlantic we employ Croll’s paradigm that glacial - interglacial fluctuations are analogous to seasonal fluctuations. Our general picture for the last 300 kyrs is that, as concerns the South Atlantic, intensity of surface water (heat) transport depends on the strength of the SE trades. From various lines of evidence it appears that stronger SE trades appeared during glacials and cold substages during interglacials, analogous to conditions in southern winter (August).

Beryllium 10, thorium 230, and protactinium 231 in Galapagos microplate sediments: Implications of hydrothermal activity and paleoproductivity changes during the last 100,000 years

Biogenic particle fluxes from highly productive surface waters, boundary scavenging, and hydrothermal activity are the main factors influencing the deposition of radionuclides in the area of the Galapagos microplate, eastern Equatorial Pacific. In order to evaluate the importance of these three processes throughout the last 100 kyr, concentrations of the radionuclides 10Be, 230Th, and 231Pa, and of Mn and Fe were measured at high resolution in sediment samples from two gravity cores KLH 068 and KLH 093. High biological productivity in the surface waters overlying the investigated area has led to 10Be and 231Pa fluxes exceeding production during at least the last 30 kyr and probably the last 100 kyr. However, during periods of high productivity at the up welling centers off Peru and extension of the equatorial high-productivity zone, a relative loss of 10Be and 231Pa may have occurred in these sediment cores because of boundary scavenging. The effects of hydrothermal activity were investigated by comparing the 230Thex concentrations to the Mn/Fe ratios and by comparing the fluxes of 230Th and 10Be which exceed production. The results suggest an enhanced hydrothermal influence during isotope stages 4 and 5 and to a lesser extent during isotope stage 1 in core KLH 093. During isotope stages 2 and 3, the hydrothermal supply of Mn was deposited elsewhere, probably because of changes in current regime or deep water oxygenation. A strong increase of the Mn/Fe ratio at the beginning of climatic stage 1 which is not accompanied by an increase of the 230Thex concentration is interpreted to be an effect of Mn remobilization and reprecipitation in the sediment.

Reconstruction of past nutrient utilization in the Eastern Angola Basin based on sedimentary 15N/14N ratios

The δ15N records of two sediment cores from the Angola Basin have been used to investigate past changes in nutrient utilization in the surface waters of this highly productive region. Low 15N/14N ratios in the cores (from >3000 m water depth) generally correspond to low sea surface temperatures and high sedimentary organic content, demonstrating a link between upwelling, productivity, and the degree of nitrate depletion. Glacial δ15N was lower by 0.5–0.7‰, and paleoproductivity was elevated by around 30% relative to interglacial periods, indicating that although productivity was higher, nitrate was less depleted during glacial than interglacial periods. More pronounced than the glacial/interglacial changes in δ15N values of both cores are precession-related (23 kyr) fluctuations. These cyclic δ15N variations, which are in phase with those of paleoproductivity and sea surface temperature, indicate that changes in trade wind driven upwelling intensity and the advection of cold, nutrient-rich water from the south drive nutrient availability and productivity off the southwest coast of Africa. No strong evidence was found from sedimentary δ15N values for denitrification in the water column in the Angola Basin during the past 180 kyr.

Impact of climatic changes on the benthic foraminiferal fauna in the Ionian Sea during the last 330,0000 years

To reconstruct paleoceanographic changes in the eastern Mediterranean during the last 330,000 years, we studied benthic foraminifera in a piston core from the Ionian Sea. The fauna exhibits large fluctuations in foraminiferal number, diversity, and species composition. Interglacials are characterized by low foraminiferal number and diversity indicating oligotrophic conditions. Directly below or above interglacial sapropels, increased numbers of low-oxygen-tolerant species indicate a strong reduction of deep water circulation. Glacials are characterized by increased foraminiferal number and diversity and faunas that are dominated by shallow infaunal species indicating mesotrophic conditions. Around glacial sapropel S6 very high foraminiferal numbers and the dominance of shallow and deep infaunal species suggest enhanced organic matter fluxes. These faunal results provide information about changes in the African and North Atlantic climate systems (monsoon and westerlies) controlling the humidity and wind stress in the Mediterranean region.

Stable nitrogen isotopes in Angola Basin surface sediments

Abstract Sedimentary δ15N contains a record of biogeochemical processes occurring in the water column. Stable nitrogen isotopes were measured in the bulk sedimentary matter of 18 surface sediment samples collected in three transects in the eastern Angola Basin, perpendicular to the coast, at 6 °S, 12 °S and 17 °S and two samples from the Zaire estuary. Relative enrichment in 15N in sediments with distance from the coast was seen in each of the three transects off northern, middle and southern Angola. Values in the Zaire Fan (northern Angola) and southern Angola increased by 1.9%. and 1.6%., respectively, from shallow (~100 m) to deep (~4500 m) water sediments and in middle Angola by 2.5%. (water depth range 73–3809 m). We propose that in the Angola region the degree of fractionation of nitrogen isotopes in organic matter is a function of nutrient supply to sunlit waters. The shift towards lighter isotopic values in shallow water signifies that the larger nitrate pool is not utilized as extensively as in water farther away from the coastal upwelling centers. Southern Angola sediments were more than 1%. lower (average δ15N, 5.4%.) than the middle and northern transects (average δ15N, 7.2%. and 6.5%., respectively). This difference is attributed to higher nitrate concentrations in the surface water at 17 °S, for which there are two reasons. One is that these sediments are located south of the Angola/Benguela front at ~16 °S which demarcates the convergence of warm equatorial water with cold, nutrient-rich Benguela Current water. The second cause is the intense Namibian coastal upwelling zone extending northward up to the front. Comparisons of bulk δ15N with δ13Corg and C N ratios show that the observed sedimentary δ15N variations with water depth are due neither to water column diagenesis nor to mixing of terrestrial material with marine-derived organic matter. The two estuary sediment samples were mainly terrigenous in origin, based on δ13Corg and C N ratios. δ15N values of these two samples were 7.0%. and 7.6%.

10Be and Ba concentrations in West African sediments trace productivity in the past

Particle reactive elements are scavenged to a higher degree at ocean margins than in the open ocean due to higher fluxes of biogenic and terrigenous particles. In order to determine the influence of these processes on the depositional fluxes of10Be and barium we have performed high-resolution measurements on sediment core GeoB 1008-3 from the Congo Fan. Because the core is dominated by terrigenous matter supplied by the Congo River, it has a high average mass accumulation rate of 6.5 cm/kyr. Biogenic10Be and Ba concentrations were calculated from total concentrations by subtracting the terrigenous components of10Be and Ba, which are assumed to be proportional to the flux of Al2O3. The meanBa/Al weight ratio of the terrigenous component was determined to be 0.0045. The unusualy high terrigenous10Be concentrations of 9.1 × 109 atoms/g Al2O3 are either due to input of particles with high10Be content by the Congo River or due to scavenging of oceanic10Be by riverine particles. The maxima of biogenic10Be and Ba concentrations coincide with maxima of the paleoproductivity rates. Time series analysis of the10Be and of Ba concentration profiles reveals a strong dominance of the precessional period of 24 kyr, which also controls the rates of paleoproductivity in this core. During the maxima of productivity the flux of biogenic Ba is enhanced to a larger extent than that of biogenic10Be. Applying a model for coastal scavenging, we ascribe the observed higher sensitivity of Ba to biogenic particle fluxes to the fact that the ocean residence time of Ba is approximately 10 times longer than that of10Be.

The South Atlantic Oxygen Isotope Record of Planktic Foraminifera

This paper reviews the recording of oxygen isotope ratios in planktic foraminifera and summarizes recent results of the application of oxygen isotopes in paleoceanographic studies of the South Atlantic. The most important factors controlling the δ18O of planktic foraminifera are temperature, the δ18O and the pH of ambient seawater. Seasonal and vertical calcification weight the mean δ18O of a foraminiferal population towards the hydrographic conditions in the preferred ecological niche. After deposition, the δ18O signal is affected by bioturbation and dissolution. Despite many influence factors, the composition of oxygen isotopes in fossil tests of planktic foraminifera provides important constraints on variations of the surface water hydrography of the South Atlantic and the Southern Ocean throughout the past 20,000 years. During the last glacial maximum, the Polar Front remained close to its modem position or shifted only slightly towards the north. In the tropics, oxygen isotopes indicate only a moderate glacial cooling of 2–3°C. During deglaciation, oxygen isotope ratios in the eastern boundary currents of the subtropical South Atlantic decreased asynchronously relative to those in the eastern North Atlantic, with the highest interhemispheric contrasts during the Younger Dryas and the Heinrich Event 1. This pattern is consistent with a redistribution of heat within the Atlantic Ocean in response to a weakening of the thermohaline circulation. The slowdown of deglacial overturning was associated with a southward displacement of the thermal equator and the Intertropical Convergence.

Seasonal variability of δ15N in sinking particles in the Benguela upwelling region

Temporal changes in δ 15 N values of sinking particles collected with sediment traps in the Benguela upwelling regime off southwest Africa mirrored variations in the input of inorganic nitrogen to the surface water. Reductions in δ 15 N (to as low as 2.5‰) corresponded to low sea surface temperatures during austral spring and late austral autumn/early winter, indicating increased nitrate availability due to the presence of recently upwelled water. High particulate fluxes accompanied the low δ 15 N values and sea surface temperatures, reflecting increased productivity, fueled by the upwelled nutrients. High δ 15 N values (up to 13.1‰) coincided with high sea surface temperatures and low particle fluxes. In this area, the seaward extension of upwelling filaments, which usually occurs twice yearly, brings nutrient-rich water to the euphotic zone and leads to elevated productivity and relatively lower δ 15 N values of the particulate nitrogen. Satellite images of ocean chlorophyll show that productivity variations coincide with δ 15 N changes. The observed isotopic pattern does not appear to have been caused by variations in the species composition of the phytoplankton assemblage. Calculations based on δ 15 N of the sinking particulate nitrogen show that the surface nitrate pool was more depleted during late austral summer/early fall and mid-winter and that supply exceeded demand during the intense spring bloom and in late austral fall. The main uncertainty associated with these estimates is the effect of diagenesis on δ 15 N and possible variability in preservation of the isotope signal between periods of high and low particle flux.

The South Atlantic Carbon Isotope Record of Planktic Foraminifera

We reviewed the paleoceanographic application of the carbon isotope composition of planktic foraminifera. Major controls on the distribution of δ13C of dissolved CO2 (δ13C∑CO2) in the modern ocean are photosynthesis-respiration cycle, isotopic fractionation during air-sea exchange, and circulation. The carbon isotope composition of surface waters is not recorded without perturbations by planktic foraminifera. Besides δ13C∑CO2 of the surrounding seawater, the δ13C composition of planktic foraminifera is affected by vital effects, the water depth of calcification and postdepositional dissolution. We compared several high-resolution (>10cm/ka) carbon isotope records from the Southern Ocean, the Benguela upwelling system, and the tropical Atlantic. In the Southern Ocean, carbon isotope values are about 1.2 %0 lower during the LGM and up to 1.7 %0 lower during the last deglaciation, when compared to the Holocene. These depletions might be explained with a combination of a subsurface nutrient enrichment and reduced air-sea exchange due to an increased stratification of surface waters. In the Benguela Upwelling system, waters originating in the south are upwelled. While the deglacial minimum is transferred and recorded in its full extent in the δ13C record of Globigerina bulloides, glacial values show only little changes. This might suggest, that the lower glacial δ13C values of high-latitude surface waters are not upwelled off Namibia, or that G. bulloides records post-upwelling conditions, when increased seasonal production has already increased surface-water δ13C. Synchronous to the δ13C depletions in high latitudes, low δ13C values were recorded in Globigerinoides sacculifer during the LGM and during the last deglaciation in the nutrient-depleted western equatorial Atlantic. Hence, part of the glacial-interglacial variability presumably transferred from high to low latitudes seems to be related to changes in thermodynamic fractionation. The variability in δ13C is lowest in the northernmost core M35003-4 from the eastern Caribbean, implying that the Antarctic Intermediate Water might have acted as a conduit to transfer the deglacial minimum to tropical surface waters.