Electromagnetic, rock magnetic, and geochemical properties of surficial sediments in Eckernförde Bay

Submarine groundwater discharge in coastal settings can massively modify the hydraulic and geochemical conditions of the seafloor. Resulting local anomalies in the morphology and physical properties of surface sediments are usually explored with seismo-acoustic imaging techniques. Controlled source electromagnetic imaging offers an innovative dual approach to seep characterization by its ability to detect pore-water electrical conductivity, hence salinity, as well as sediment magnetic susceptibility, hence preservation or diagenetic alteration of iron oxides. The newly developed electromagnetic (EM) profiler Neridis II successfully realized this concept for a first time with a high-resolution survey of freshwater seeps in Eckernförde Bay (SW Baltic Sea). We demonstrate that EM profiling, complemented and validated by acoustic as well as sample-based rock magnetic and geochemical methods, can create a crisp and revealing fingerprint image of freshwater seepage and related reductive alteration of near-surface sediments. Our findings imply that (1) freshwater penetrates the pore space of Holocene mud sediments by both diffuse and focused advection, (2) pockmarks are marked by focused freshwater seepage, underlying sand highs, reduced mud thickness, higher porosity, fining of grain size, and anoxic conditions, (3) depletion of Fe oxides, especially magnetite, is more pervasive within pockmarks due to higher concentrations of organic and sulfidic reaction partners, and (4) freshwater advection reduces sediment magnetic susceptibility by a combination of pore-water injection (dilution) and magnetite reduction (depletion). The conductivity vs. susceptibility biplot resolves subtle lateral litho- and hydrofacies variations.

Western Arabian Sea SST estimation

Millennial-scale records of planktonic foraminiferal Mg/Ca, bulk sediment UK37', and planktonic foraminiferal d18O are presented across the last two deglaciations in sediment core NIOP929 from the Arabian Sea. Mg/Ca-derived temperature variability during the penultimate and last deglacial periods falls within the range of modern day Arabian Sea temperatures, which are influenced by monsoon-driven upwelling. The UK37'-derived temperatures in MIS 5e are similar to modern intermonsoon values and are on average 3.5°C higher than the Mg/Ca temperatures in the same period. MIS 5e UK37' and Mg/Ca temperatures are 1.5°C warmer than during the Holocene, while the UK37'-Mg/Ca temperature difference was about twice as large during MIS 5e. This is surprising as, nowadays, both proxy carriers have a very similar seasonal and depth distribution. Partial explanations for the MIS 5e UK37'-Mg/Ca temperature offset include carbonate dissolution, the change in dominant alkenone-producing species, and possibly lateral advection of alkenone-bearing material and a change in seasonal or depth distribution of proxy carriers. Our findings suggest that (1) Mg/Ca of G. ruber documents seawater temperature in the same way during both studied deglaciations as in the present, with respect to, e.g., season and depth, and (2) UK37'-based temperatures from MIS 5 (or older) represent neither upwelling SST nor annual average SST (as it does in the present and the Holocene) but a higher temperature, despite alkenone production mainly occurring in the upwelling season. Further we report that at the onset of the deglacial warming, the Mg/Ca record leads the UK37' record by 4 ka, of which a maximum of 2 ka may be explained by postdepositional processes. Deglacial warming in both temperature records leads the deglacial decrease in the d18O profile, and Mg/Ca-based temperature returns to lower values before d18O has reached minimum interglacial values. This indicates a substantial lead in Arabian Sea warming relative to global ice melting.

Sea-surface reconstruction of the east-equatorial South Atlantic

In order to reconstruct Late Quatemary variations of surface oceanography in the eastequatorial South Atlantic, time series of sea-surface temperatures (SST) and paleoproductivity were established from cores recovered in the Guinea and Angola Basins, and at the Walvis Ridge. These records, based on sedimentary alkenone and organic carbon concentrations, reveal that during the last 350,000 years surface circulation and productivity changes in the east-equatorial South Atlantic were highiy sensitive to climate forcing at 23- and 100-kyr periodicities. Covarying SST and paleoproductivity changes at the equator and at the Walvis Ridge appear to be driven by variations in zonal trade-wind intensity, which forces intensification or reduction of coastal and equatorial upwelling, as well as enhanced Benguela cold water advection from the South. Phase relationships of precessional variations in the paleoproductivity and SST records from the distinct sites were evaluated with respect to boreal summer insolation over Africa, movements of southem ocean thermal fronts, and changes in global ice volume. The 23-kyr phasing implies a sensitivity of eastem South Atlantic surface water advection and upwelling to West African monsoon intensity and to changes in the position ofthe subtropical high pressure cell over the South Atlantic, both phenomena which modulate zonal strength of southeasterly trades. SST and productivity changes north of 20°S lack significant variance at the 41-kyr periodicity; and at the Walvis Ridge and the equator lead changes in ice volume. This may indicate that obliquity-driven clirnate change, characteristic for northem high latitudes, e.g fluctuations in continental ice masses, did not substantially influence subtropical and tropical surface circulation in the South Atlantic. At the 23-kyr cycle SST and productivity changes in the eastern Angola Basin lag those in the equatorial Atlantic and at the Walvis Ridge by about 3500 years. This lag is explained by variations in cross-equatorial surface water transport and west-east countercurrent retum flow modifying precessional variations of SST and productivity in the eastem Angola Basin relative to those in the mid South Atlantic area under the central field of zonal trade winds. Sea level-related shifts of upwelling cells in phase with global clirnate change may be also recorded in SST and productivity variability along the continental margin off Southwest Africa. They may account for the delay of the paleoceanogreaphic signal from continental margin sites with respect to that from the pelagic sites at the equator and the Walvis Ridge.

Biomass of seston in the benthic layer of the Southwestern Indian Ocean

At stations to 1530 m depth in the Mozambique Channel and on the Saya-de-Malha and Walters banks seston biomass 2 m above the bottom was lower than at 30 m. Above the Walters shoal this difference was 13.2 mg/m**3 and was not equal to zero for P < 0.001. These results contradict previous ideas of biomass increase in benthic layers. The most likely cause of the observed impoverishment of plankton may be predominant consumption of living zooplankton component of seston by bottom and near-bottom predators. In the area of the Walters shoal this consumption is estimated as being about 300 mg/m**2 per day. Animals inhabiting this area live mainly on plankton brought in by horizontal advection, so that existence of faunal assemblages even on shallow-water submarine elevations is supported not mainly by local photosynthesis, but by primary production of surrounding waters.

(Table DR1) Biogenic silica and chert in the Pacific Ocean, DSDP and ODP data

Evidence for the dissolution of biogenic silica at the base of pelagic sections supports the hypothesis that much of the chert formed in the Pacific derives from the dissolution and reprecipitation of this silica by hydrothermal waters. As ocean bottom waters flow into and through the crust, they become warmer. Initially they remain less saturated with respect to dissolved silica than pore water in the overlying sediments. With the diffusion of heat, dissolved ions, and to some extent the advection of water itself, biogenic silica in the basal part of the sedimentary section is dissolved. Upon conductively cooling, these pore waters precipitate chert layers. The most common thickness for the basal silica-free zone (20 m) lies below the most common height of the top of the chert interval above basement (50 m). This mode of chert formation explains the frequent occurrence of chert layers at very shallow subbottom depths in pelagic sections of the Pacific. It is also consistent with the common occurrence of cherts

Stable isotope record of ODP Sites 108-658 and 108-659

Ocean Drilling Program Site 658, cored below a major upwelling cell offshore Cap Blanc, contains a largely undisturbed hemipelagic sediment section spanning the Brunhes Chron and the early Quaternary and late Pliocene. The companion Site 659 recovered a complete and undisturbed Neogene profile further offshore that serves as a nonupwelling pelagic reference section. Oxygen and carbon isotope ratios in benthic (C. wuellerstorfi and in part Uvigerina sp.) and planktonic foraminifers (G. inflata) provide a climatic record of high resolution for the Brunhes Chron. At Site 658 the record extends back to the early Pleistocene and late Pliocene. The standard oxygen isotope record of the last 730,000 yr is markedly refined by a well-documented high-frequency variation (e.g., by a new "aborted" ice age at stage 13.2 and by Younger-Dryas style climatic setbacks during most terminations). In the late Pliocene, the numerical oxygen isotope stage taxonomy was extended back to stage 137 about 3.3 Ma ago. In comparison with published records, stage 114 at 2.7 Ma represents the first major glaciation event, when 18O was short-term enriched up to a middle Pleistocene glacial d18O level. About 3.17 Ma ago (stage 133), the interglacial oxygen isotope values of C. wuellerstorfi started to increase by 0.5 per mil until 2.7 Ma and then remained largely constant until the Holocene. Based on the d13C difference between C. wuellerstorfi and G. inflata, the dissolved CO2 in the ambient bottom water of Site 658 was dominated by the flux of particulate carbon from the overlying upwelling cell during the last 630,000 yr. In contrast, the advection of (upper) North Atlantic Bottom Water dominated in the control of the local CO2 content during the early Pleistocene and late Pliocene.

Ocean temperature response to idealized Gleissberg and de Vries solar cycles in a comprehensive climate model

The ~90-year Gleissberg and ~200-year de Vries cycles have been identified as two distinctive quasi-periodic components of Holocene solar activity. Evidence exists for the impact of such multi-decadal to centennial-scale variability in total solar irradiance (TSI) on climate, but concerning the ocean, this evidence is mainly restricted to the surface response. Here we use a comprehensive global climate model to study the impact of idealized solar forcing, representing the Gleissberg and de Vries cycles, on global ocean potential temperature at different depth levels, after a recent proxy record indicates a signal of TSI anomalies in the northeastern Atlantic at mid-depth. Potential impacts of TSI anomalies on deeper oceanic levels are climatically relevant due to their possible effect on ocean circulation by altering water mass characteristics. Simulated solar anomalies are shown to penetrate the ocean down to at least deep-water levels. Despite the fact that the two forcing periods differ only by a factor of ~2, the spatial pattern of response is significantly distinctive between the experiments, suggesting different mechanisms for solar signal propagation. These are related to advection by North Atlantic Deep Water flow (200-year forcing), and barotropic adjustment in the South Atlantic in response to a latitudinal shift of the westerly wind belt (90-year forcing).

Middle Miocene to recent sedimentological record of ODP sites 177-1088 and 177-1092

During Leg 177 of the Ocean Drilling Program (ODP), well-preserved Middle Miocene to Pleistocene carbonate-rich sediment records were recovered on a north-south transect through the south-eastern Atlantic sector of the Southern Ocean at Site 1088 on the Agulhas Ridge and Site 1092 on Meteor Rise. Both sites were dominated by the deposition of calcareous nannofossil oozes through the Miocene, indicating low biological productivity in warm to temperate surface waters. A continuous increase in the proportions of foraminifera since the latest Miocene (6.5 Ma) points to enhanced nutrient supply, possibly related to the global 'biogenic bloom' event across the Miocene-Pliocene boundary. Since the Late Pliocene, different styles of biological productivity developed between the sites. Enhanced deposition of biosiliceous constituents at the southern Site 1092, particularly in the Early Pleistocene, is consistent with the formation of the Circum-Antarctic Opal Belt since 2.5 Ma in a setting near the Polar Front, whereas carbonate deposition still prevailed at the northern Site 1088 situated near the Subtropical Front. Clay-mineral tracers of water-mass advection together with the pattern of sedimentation rates and hiatuses reflect distinct pulses in the development of regional ocean circulation between 14 and 12 Ma, around 8 Ma and since 2.8 Ma. These pulses can be related to Antarctic ice-sheet extension that mediates the production and flow of southern source water, and stepwise increases in North Atlantic Deep Water production that drives global conveyor circulation. At Site 1088, illite chemistry and silt/clay ratios of the terrigenous sediment fraction reflect the history of terrestrial climate in southern Africa, with humid conditions prior to the Early Late Miocene (9.7 Ma), followed by a dry episode until 7.7 Ma. The latest Miocene and Early Pliocene were characterized by a humid episode until modern aridity was established in the Late Pliocene between 4.0 and 2.8 Ma. These climate changes were related to the latitudinal migration of climate belts in response to tectonically caused reorganizations in atmospheric and ocean circulation.

Age determination, planktic foraminifera and stable istopes of sediment core MSM05/5_712-2

A sediment core from the West Spitsbergen continental margin was studied to reconstruct climate and paleoceanographic variability during the last ~9 ka in the eastern Fram Strait. Our multiproxy evidence suggests that the establishment of the modern oceanographic configuration in the eastern Fram Strait occurred stepwise, in response to the postglacial sea-level rise and the related onset of modern sea-ice production on the shallow Siberian shelves. The late Early and Mid Holocene interval (9 to 5 ka) was generally characterized by relatively unstable conditions. High abundance of the subpolar planktic foraminifer species Turborotalita quinqueloba implies strong intensity of Atlantic Water (AW) inflow with high productivity and/or high AW temperatures, resulting in a strong heat flux to the Arctic. A series of short-lived cooling events (8.2, 6.9. and 6.1 ka) occurred superimposed on the warm late Early and Mid Holocene conditions. Our proxy data imply that simultaneous to the complete postglacial flooding of Arctic shallow shelves and the initiation of modern sea-ice production, strong advance of polar waters initiated modern oceanographic conditions in the eastern Fram Strait at ~5.2 ka. The Late Holocene was marked by the dominance of the polar planktic foraminifer species Neogloboquadrina pachyderma, a significant expansion of sea ice/icebergs, and strong stratification of the water column. Although planktic foraminiferal assemblages as well as sea surface and subsurface temperatures suggest a return of slightly strengthened advection of subsurface Atlantic Water after 3 ka, a relatively stable cold-water layer prevailed at the sea surface and the study site was probably located within the seasonally fluctuating marginal ice zone during the Neoglacial period.

Analysis of an ancient global warming event, the Paleocene-Eocene thermal maximum, under examination from ODP Holes 113-689B and 113-690B

Spatiotemporal patterns of carbonate dissolution provide a critical constraint on carbon input during an ancient (~55.5 Ma) global warming event known as the Paleocene-Eocene thermal maximum (PETM), yet the magnitude of lysocline shoaling in the Southern Ocean is poorly constrained due to limited spatial coverage in the circum-Antarctic region. This shortcoming is partially addressed by comparing patterns of carbonate sedimentation at the Site 690 PETM reference section to those herein reconstructed for nearby Site 689. Biochemostratigraphic correlation of the two records reveals that the first ~36 ka of the carbon isotope excursion (CIE) signaling PETM conditions is captured by the Site 689 section, while the remainder of the CIE interval and nearly all of the CIE recovery are missing due to a coring gap. A relatively expanded stratigraphy and higher carbonate content at mid-bathyal Site 689 indicate that dissolution was less severe than at Site 690. Thus, the bathymetric transect delimited by these two PETM records indicates that the lysocline shoaled above Site 689 (~1,100 m) while the calcite compensation depth remained below Site 690 (~1,900 m) in the Weddell Sea region. The ensuing recovery of carbonate sedimentation conforms to a bathymetric trend best explained by gradual lysocline deepening as negative feedback mechanisms neutralized ocean acidification. Further, biochemostratigraphic evidence indicates the tail end of the CIE recovery interval at both sites has been truncated by a hiatus most likely related to vigorous production and advection of intermediate waters.

Chemical and isotope compositions of bottom sediments from ODP Leg 168, Eastern Flank of the Juan de Fuca Ridge

Ten sites were drilled in the eastern flank of the Juan de Fuca Ridge (North East Pacific) along a 100 km-long east-west transect during Leg ODP 168. This study focuses on the mineralogical and chemical study of sediments that overly basaltic basement through which seawater circulates. Silicate authigenesis was observed in the sediment layer just above basement at sites located more than 30 km from the ridge axis. This sediment alteration is particularly abundant at ODP Sites 1031 and 1029 where authigenic formation of Fe-Mg rich smectite and zeolite and the dissolution of biogenic calcite are observed. Comparison of the distribution of the alteration in the basal sediment collected along this transect suggests that diffusional transport of aqueous solutes from the basement into the overlying sediment cannot produce the mineralogical and chemical changes in the basal sediments at Sites 1031 located on a basement topographic high, and at Site 1029 located at about 50 km from the ridge axis on a buried basement area. Vertical advection of basement fluid though the sediment section is required to produce this alteration. These processes are still active at Site 1031, based on systematic variations in pore-water profiles and temperatures obtained from stable isotopic data on calcium carbonates and the nature of authigenic minerals. At Site 1029, there is no present-day advection of basement fluids though the sediment section, suggesting that this is a relic site for fluid flow.

Monthly interpolated coral d18O records from Bonaire corals

Several proxy-based and modeling studies have investigated long-term changes in Caribbean climate during the Holocene, however, very little is known on its variability on short timescales. Here we reconstruct seasonality and interannual to multidecadal variability of sea surface hydrology of the southern Caribbean Sea by applying paired coral Sr/Ca and d18O measurements on fossil annually banded Diploria strigosa corals from Bonaire. This allows for better understanding of seasonal to multidecadal variability of the Caribbean hydrological cycle during the mid- to late Holocene. The monthly resolved coral Delta d18O records are used as a proxy for the oxygen isotopic composition of seawater (d18Osw) of the southern Caribbean Sea. Consistent with modern day conditions, annual d18Osw cycles reconstructed from three modern corals reveal that freshwater budget at the study site is influenced by both net precipitation and advection of tropical freshwater brought by wind-driven surface currents. In contrast, the annual d18Osw cycle reconstructed from a mid-Holocene coral indicates a sharp peak towards more negative values in summer, suggesting intense summer precipitation at 6 ka BP (before present). In line with this, our model simulations indicate that increased seasonality of the hydrological cycle at 6 ka BP results from enhanced precipitation in summertime. On interannual to multidecadal timescales, the systematic positive correlation observed between reconstructed sea surface temperature and salinity suggests that freshwater discharged from the Orinoco and Amazon rivers and transported into the Caribbean by wind-driven surface currents is a critical component influencing sea surface hydrology on these timescales.

Geochemistry, fluxes and nitrogen isotopic composition data of traps LP1 and EBC2-1 and cores GeoB4242-4 and Geo4301-1

We compare total and biogenic particle fluxes and stable nitrogen isotope ratios (d15N) at three mooring sites along a productivity gradient in the Canary Islands region with surface sediment accumulation rates and sedimentary d15N. Higher particle fluxes and sediment accumulation rates, and lower d15N were observed in the upwelling influenced eastern boundary region (EBC) compared to the oligotrophic sites north of Gran Canaria [European Station for Time-Series in the Ocean, Canary Islands (ESTOC]] and north of La Palma (LP). The impact of organic matter degradation and lateral particle advection on sediment accumulation was quantified with respect to the multi-year flux record at the ESTOC. Remineralisation of organic matter in the water column and at the sediment surface resulted in an organic carbon preservation of about 0.8% and total nitrogen preservation of about 0.4% of the estimated export production. Higher total and carbonate fluxes and accumulation rates in the lower traps and surface sediment compared to the upper traps indicated that at least 50% of the particulate matter at the ESTOC was derived from allochthonous sources. Low d15N values in the lower traps of the ESTOC and LP point to a source region influenced by coastal upwelling. We conclude from this study that the reconstruction of export production or nutrient regimes from sedimentary records in regions with strong productivity gradients might be biased due to the mixture of particles originating from autochthonous and allochthonous sources. This could result in an imprint of high productivity signatures on sedimentation processes in oligotrophic regions.