Gerhard Bönisch

The global spectrum of plant form and function

Earth is home to a remarkable diversity of plant forms and life histories, yet comparatively few essential trait combinations have proved evolutionarily viable in today’s terrestrial biosphere. By analysing worldwide variation in six major traits critical to growth, survival and reproduction within the largest sample of vascular plant species ever compiled, we found that occupancy of six-dimensional trait space is strongly concentrated, indicating coordination and trade-offs. Three-quarters of trait variation is captured in a two-dimensional global spectrum of plant form and function. One major dimension within this plane reflects the size of whole plants and their parts; the other represents the leaf economics spectrum, which balances leaf construction costs against growth potential. The global plant trait spectrum provides a backdrop for elucidating constraints on evolution, for functionally qualifying species and ecosystems, and for improving models that predict future vegetation based on continuous variation in plant form and function.

Meteorological processes forcing Saharan dust emission inferred from MSG-SEVIRI observations of subdaily dust source activation and numerical models

Fifteen-minute Meteosat Second Generation (MSG) Spinning Enhanced Visible and Infrared Imager (SEVIRI) infrared dust index images are used to identify dust source areas. The observations of dust source activation (DSA) are compiled in a 1° × 1° map for the Sahara and Sahel, including temporal information at 3-hourly resolution. Here we use this data set to identify the most active dust source areas and the time of day when dust source activation occurs most frequently. In the Sahara desert 65% of DSA (March 2006 to February 2008) occurs during 0600-0900 UTC, pointing toward an important role of the breakdown of the nocturnal low-level jet (LLJ) for dust mobilization. Other meteorological mechanisms may lead to dust mobilization including density currents initiated by deep convective systems which mobilize dust fronts (haboobs) occurring preferentially in the afternoon hours and cyclonic activities. The role of the nocturnal LLJ for dust mobilization in the Sahara is corroborated by regional model studies and analysis of meteorological station data.

Arctic river-runoff: mean residence time on the shelves and in the halocline

The mean residence time of river-runoff on the shelves and in the halocline of the Arctic Ocean is estimated from salinity and tracer data (tritium, 3He and the 18O/16O ratio). These estimates are derived from comparison of apparent tracer ages of the halocline waters using a combination of tracers that yield different information: (1) the tritium “vintage” age, which records the time that has passed since the river-runoff entered the shelf; and (2) the tritium/3He age, which reflects the time since the shelf waters left the shelf. The difference between the ages determined by these two methods is about 3–6 years. Correction for the initial tritium/3He age of the shelf waters (about 0.5–1.5 years) yields a mean residence time of the river-runoff on the shelves of the Siberian Seas of about 3.5 ± 2 years.

Reduction of deepwater formation in the Greenland Sea during the 1980s: Evidence from tracer data

Hydrographic observations and measurements of the concentrations of chlorofluorocarbons (CFCs) have suggested that the formation of Greenland Sea Deep Water (GSDW) slowed down considerably during the 1980s. Such a decrease is related to weakened convection in the Greenland Sea and thus could have significant impact on the properties of the waters flowing over the Scotland-Iceland-Greenland ridge system into the deep Atlantic. Study of the variability of GSDW formation is relevant for understanding the impact of the circulation in the European Polar seas on regional and global deep water characteristics. New long-term multitracer observations from the Greenland Sea show that GSDW formation indeed was greatly reduced during the 1980s. A box model of deepwater formation and exchange in the European Polar seas tuned by the tracer data indicates that the reduction rate of GSDW formation was about 80 percent and that the start date of the reduction was between 1978 and 1982.

Renewal and circulation of intermediate waters in the Canadian Basin observed on the SCICEX 96 cruise

During the summer of 1996 the nuclear submarine USS Pogy occupied a line of stations extending through the middle of the Canadian Basin between about 88°N, 44°W (Lomonosov Ridge) and about 78°N, 144°W (center of the Canada Basin). CTD/Niskin bottle casts extending to 1600 m were carried out at eight stations, providing the first high-quality temperature, salinity, CFC, tritium, and 3He data obtained from this region, although XCTD data had previously been collected in this region. These data, along with data from stations at the basin boundary to the south and west, reveal the presence of well-ventilated intermediate water beneath the halocline in the center of the Canada Basin, indicating renewal times of the order of 1–2 decades. The least ventilated intermediate water was observed at the northern end of the Canada Basin along the southern flank of the Alpha Ridge. Intermediate water is derived from the Atlantic Ocean and enters the Arctic Ocean through Fram Strait and the Barents Sea. It flows around the Arctic basins in boundary currents and splits in the eastern Amundsen Basin with one branch crossing the Lomonosov Ridge and flowing along the East Siberian continental slope and the other flowing along the Eurasian flank of the Lomonosov Ridge. From the 1996 Scientific Ice Expedition (SCICEX 96) observations we conclude that the branch that flows along the East Siberian continental slope transports this water to the Chukchi Rise, where it apparently enters the central Canada Basin with some flow continuing along the boundary to the southern Canada Basin. The Fram Strait Branch Water mixes extensively with waters from the Canadian Basin during its transit along the East Siberian continental slope, being diluted by a factor of about 5 by the time it reaches the central Canada Basin. The Barents Sea Branch Water does not undergo such extensive mixing and is diluted by a factor of only about 2 when it reaches the central Canada Basin.

Deep water formation and exchange rates in the Greenland/Norwegian Seas and the Eurasian Basin of the Arctic Ocean derived from tracer balances

Abstract Multi-tracer data sets collected in the Greenland/Norwegian seas and the Eurasian Basin of the Arctic Ocean in the 1970s and 1980s are used, together with temperature and salinity, to (1) constrain box model calculations of the deep water formation rates in the Greenland Sea and the Eurasian Basin of the Arctic Ocean, and (2) estimate the exchange rates of deep waters (depth ≥1,500m) between the Greenland/Norwegian Seas and the Eurasian Basin. We obtain deep water formation rates of 0.1Sv (since 1980) to 0.47Sv (from at least 1965 to 1980) for the Greenland Sea, and 0.3Sv for the Eurasian Basin of the Arctic Ocean. The southward flux of Eurasian Basin Deep Water through Fram Strait is estimated to be about 1Sv. About 0.12Sv of this flux are transported into the Greenland Sea, about 0.37Sv reach the deep Norwegian Sea through the Jan Mayen Fracture Zone, and about 0.39Sv leave the Arctic Ocean through a shallower core which more or less directly feeds into the Iceland Sea, and, after modification, eventually ends up in the overflow waters. The outflow of Eurasian Basin Deep Water is balanced by deep water formation in the Arctic Ocean and by inflow of Norwegian Sea Deep water. About 0.77Sv of deep water formed in the Greenland Sea and the Eurasian Basin contribute to the formation of North Atlantic Deep Water. Uncertainties of the fluxes are estimated to be roughly ±20 to 30%.

Long‐term trends of temperature, salinity, density, and transient tracers in the central Greenland Sea

We present long-term observations of temperature, salinity, tritium/ 3 He, chlorofluorocarbon-11 (CFC 11), and chlorofluorocarbon-12 (CFC12) for the central Greenland Gyre. The time series span the periods between 1952 and 1994 (temperature), 1981 and 1994 (salinity), 1972 and 1994 (tritium/ 3 He), and 1982 and 1994 (CFCs). The correlation between hydrographic and transient tracer data indicates that low temperatures in the deep water in the early 1950s and between 1960 and 1980 reflect periods of higher deep water formation rates whereas periods of increasing temperatures in the late 1950s and between 1980 and 1994 are related to low deep water formation rates. However, the transient tracer observations obtained in the 1980s and early 1990s indicate that even during periods of low deep water formation, some water from the upper water column contributed to deep water formation between 1980 and 1994. In 1994, the deep water reached temperatures and salinities of -1.149 °C and 34.899, respectively, and no longer fits most of the classical definitions of Greenland Sea Deep Water (-1.29°C< Θ < -1.0°C, 34.88 < S < 34.90). The temperature increase in the water column between 200 and 2000m depth between 1980 and 1994 corresponds to an average heating rate of about 5W m -2 over this period, resulting in a decrease in density. The 13-year warming could be balanced by intensive cooling in two winters. The surface salinity steadily increased from 34.50 in 1991 to 34.85 in 1994.

THE FIRST TRANS-ARCTIC 14C SECTION: COMPARISON OF THE MEAN AGES OF THE DEEP WATERS IN THE EURASIAN AND CANADIAN BASINS OF THE ARCTIC OCEAN

We present Δ14C data collected during three cruises to the Arctic Ocean that took place in the summers of 1987 (POLARSTERN cruise ARK IV/3), 1991 (ARCTIC 91 Expedition), and 1994 (Arctic Ocean Section 94). The cruise tracks of these three expeditions cover all major basins of the Arctic Ocean (Nansen, Amundsen, Makarov and Canada basins), and can be combined to a trans-Arctic section reaching from the Barents Sea slope to the southern Canada Basin just north of Bering Strait. The section is based on 17 stations covering the entire water column (about 250 data points). The combined Δ14C data set was produced from a mixture of large volume samples measured by low-level counting and small volume samples measured by Accelerator Mass Spectrometry (AMS). We use the Δ14C section, together with previously published Δ14C data from single stations located in several basins of the Arctic Ocean, to derive mean “ages” (isolation times) of the deep waters in the Arctic Ocean. We estimate these mean “ages” to be ≈ 250 years in the bottom waters of the Eurasian Basin and ≈ 450 years in the Canadian Basin Deep Water. A remarkable feature of the Δ14C section is the homogeneity in the 14C distribution observed in the deep Canadian Basin. Within the measurement precision of about ±2‰ (LV) to about ±5‰ (AMS), we cannot detect significant horizontal or vertical Δ14C gradients below 2000 m depth between the northern boundary of the Makarov Basin and the southern margin of the Canada Basin. There is no statistically significant difference between samples measured by AMS and by low-level counting.

Mid 1980s distribution of tritium, 3He, 14C, and 39Ar in the Greenland/Norwegian seas and the Nansen Basin of the Arctic Ocean

Abstract The distributions of tritium 3 He , 14C and 39Ar observed in the period between 1985 and 1987 in the Greenland/Norwegian Seas and the Nansen Basin of the Arctic Ocean are presented. The data are used to outline aspects of the large-scale circulation and the exchange of deep water between the Greenland/Norwegian Seas and the Nansen Basin. Additionally, semi-quantitative estimates of mean ages of the main water masses found in these regions are obtained. Apparent tritium 3 He ages of the upper waters (depth 1,500m depth) of the Greenland/Norwegian Seas show apparent tritium 3 He ages between about 17 years in the Greenland Sea and 30 years in the Norwegian Sea. 39Ar based estimates of the Nansen Basin intermediate, deep and bottom water ages are 91+26−23, 161+50−44 and 277+33−31 years for Arctic Intermediate Water (AIW), Eurasian Basin Deep Water (EBDW) and Eurasian Basin Bottom Water (EBBW), respectively. Within the errors, age estimates of EBDW and EBBW based on 14 C tritium correlations are consistent with those derived from 39Ar (163 to 287 years for EBDW and 244 to 368 years for EBBW). A quantitative evaluation of the data in terms of deep water formation and exchange rates based on box model calculations is presented in an accompanying paper.

On the 14C and 39Ar distribution in the central Arctic Ocean: implications for deep water formation

We present ΔA 14 C and 39 Ar data collected in the Nansen, Amundsen and Makarov basins during two expeditions to the central Arctic Ocean (RV Polarstern cruises ARK IV/3, 1987 and ARK VIII/3, 1991). The data are used, together with published Δ 14 C values, to describe the distribution of Δ 14 C in all major basins of the Arctic Ocean (Nansen, Amundsen, Makarov and Canada Basins), as well as the 39 Ar distribution in the Nansen Basin and the deep waters of the Amundsen and Makarov Basins. From the combined Δ 14 C and 39 Ar distributions, we derive information on the mean “isolation ages” of the deep and bottom waters of the Arctic Ocean. The data point toward mean ages of the bottom waters in the Eurasian Basin (Nansen and Amundsen Basins) of ca. 250-300 yr. The deep waters of the Amundsen Basin show slightly higher 3 H concentrations than those in the Nansen Basin, indicating the addition of a higher fraction of water that has been at the sea surface during the past few decades. Correction for the bomb 14 C added to the deep waters along with bomb 3 H yields isolation ages for the bulk of the deep and bottom waters of the Amundsen Basin similar to those estimated for the Nansen Basin. This finding agrees well with the 39 Ar data. Deep and bottom waters in the Canadian Basin (Makarov and Canada Basins) are very homogeneous, with an isolation age of ca. 450 yr. Δ 14 C and 39 Ar data and a simple inverse model treating the Canadian Basin Deep Water (CBDW) as one well-mixed reservoir renewed by a mixture of Atlantic Water (29%), Eurasian Basin Deep Water (69%) and brine-enriched shelf water (2%) yield a mean residence time of CBDW of ca. 300 yr.