Geochemistry of basalts at DSDP Leg 73 Holes

During Deep Sea Drilling Project Leg 73 (South Atlantic), basaltic pillow lava, flows, and sills were encountered in Holes 519A, 520, 522B, and 524. Paleomagnetic data indicate that the basalts from Holes 519A (magnetic Anomaly 51) and 522B (Anomaly 16) have ages of about 12 m.y. and about 38 m.y., respectively. The major- and trace- (including rare-earth-) element characteristics of the Hole 519A basalts (a total of 27 m) demonstrate that these basalts are typical normal-type mid-ocean-ridge basalts (N-type MORB). In composition the basalts overlap olivine tholeiites from other normal Mid-Atlantic Ridge segments. Both the spectra of incompatible, or less-hygromagmatophile elements (such as Ti, V, Y, and Zr) and REE abundances indicate that these basalts are the result of a low-pressure fractionation of olivine, spinel, and Plagioclase prior to eruption. In Hole 520 only 1.7 m of basalt were recovered from a total drilling depth of 10.5 m. These pillow basalts crystallized from fairly evolved (N-type MORB) tholeiitic melts. In total, 19 m of basaltic pillow lavas and flows were penetrated in Hole 522B. Thirteen cooling units were distinguished on the basis of glassy margins and fine quench textures. In contrast to Holes 519A and 520, the basalts of the Hole 522B ridge section can be divided into two major groups of tholeiites: (1) Cooling Units 1 through 12 and (2) Cooling Unit 13. The basalts in this ridge section are also N-type MORBs but are generally more differentiated than those of Holes 519A and 520. The lowermost basalts (Cooling Unit 13) have the most primitive composition and make up a compositional group distinct from the more evolved basalts in the twelve units above it. Hole 524 was drilled on the south flank of the Walvis Ridge and thus provided samples from a more complex part of the South Atlantic seafloor. Three different basaltic rock suites, interlayered with volcanic detrital sediments, were encountered. The rock suites are, from top to bottom, an alkali basaltic pillow lava; a 16-m-thick alkaline diabase sill with an age of about 65 m.y. (according to K-Ar dating and planktonic foraminifers); and a second sill that is approximately 9 m thick, about 74 m.y. in age, and tholeiitic in composition, thus contrasting strongly with the overlying alkaline rocks. The alkali basalts of Hole 524 show chemical characteristics that are very similar to the basaltic lavas of the Tristan da Cunha group volcanoes, which are located approximately 400 km east of the Mid-Atlantic Ridge crest. Thus, the Walvis Ridge may plausibly be interpreted as a line of hot-spot alkaline volcanoes.

Arctic Siberia: Modern pollen dataset and pollen spectra changes over the last 7,000 years from a lacustrine sediment core (southern Taymyr region)

Palaeoecological investigations in the larch forest-tundra ecotone in northern Siberia have the potential to reveal Holocene environmental variations, which likely have consequences for global climate change because of the strong high-latitude feedback mechanisms. A sediment core, collected from a small lake (radius ~100 m), was used to reconstruct the development of the lake and its catchment as well as vegetation and summer temperatures over the last 7100 calibrated years. A multi-proxy approach was taken including pollen and sedimentological analyses. Our data indicate a gradual replacement of open larch forests by tundra with scattered single trees as found today in the vicinity of the lake. An overall trend of cooling summer temperature from a ~2 °C warmer-than-present mid-Holocene summer temperatures until the establishment of modern conditions around 3000 years ago is reconstructed based on a regional pollen-climate transfer function. The inference of regional vegetation changes was compared to local changes in the lake's catchment. An initial small water depression occurred from 7100 to 6500 cal years BP. Afterwards, a small lake formed and deepened, probably due to thermokarst processes. Although the general trends of local and regional environmental change match, the lake catchment changes show higher variability. Furthermore, changes in the lake catchment slightly precede those in the regional vegetation. Both proxies highlight that marked environmental changes occurred in the Siberian forest-tundra ecotone over the course of the Holocene.

Pollen spectra and relative pollen productivity estimates for common taxa of the northern Siberian Arctic

Pollen productivity estimates (PPE) are used to quantitatively reconstruct variations in vegetation within a specific distance of the sampled pollen archive. Here, for the first time, PPEs from Siberia are presented. The study area (Khatanga region, Krasnoyarsk territory, Russia) is located in the Siberian Sub-arctic where Larixis the sole forest-line forming tree taxon. Pollen spectra from two different sedimentary environments, namely terrestrial mosses (n=16) and lakes (n=15, median radius ~100 m) and their surrounding vegetation were investigated to extract PPEs. Our results indicate some differences in pollen spectra between moss and lake pollen. Larix and Cyperaceae for example obtained higher representation in the lacustrine than in terrestrial moss samples. This highlights that in calibration studies modern and fossil dataset should be of similar sedimentary origin. The results of the Extended R-Value model were applied to assess the relevant source area of pollen (RSAP) and to calculate the PPEs for both datasets. As expected, the RSAP of the moss samples was very small (about 10 m) compared to the lacustrine samples (about 25 km). Calculation of PPEs for the six most common taxa yielded generally similar results for both datasets. Relative to Poaceae (reference taxon, PPE=1) Betula nana-type (PPEmoss: 1.8, PPElake: 1.8) and Alnusfruticosa-type (PPEmoss: 6.4, PPElake: 2.9) were overrepresented while Cyperaceae (PPEmoss: 0.5, PPElake: 0.1), Ericaceae (PPEmoss: 0.3, PPElake <0.01), Salix (PPEmoss: 0.03, PPElake <0.01) and Larix (PPEmoss <0.01, PPElake: 0.2) were under-represented in the pollen spectra compared to the vegetation in the RSAP. The estimation for the dominant tree in the region, Larixgmelinii, is the first published result for this species, but need to be considered very preliminary. The inferred sequence from over- to under-representation is mostly consistent with results from Europe; however, still the absolute values show some differences. Gathering vegetation data was limited by flowering season and low resolute satellite imagery and accessibility of the remote location of our study area. Therefore, our estimate may serve as first reference to strengthen future vegetation reconstructions in this climate-sensitive region.