Lovisa Zillén

Hypoxia Is Increasing in the Coastal Zone of the Baltic Sea

Hypoxia is a well-described phenomenon in the offshore waters of the Baltic Sea with both the spatial extent and intensity of hypoxia known to have increased due to anthropogenic eutrophication, however, an unknown amount of hypoxia is present in the coastal zone. Here we report on the widespread unprecedented occurrence of hypoxia across the coastal zone of the Baltic Sea. We have identified 115 sites that have experienced hypoxia during the period 1955–2009 increasing the global total to ca. 500 sites, with the Baltic Sea coastal zone containing over 20% of all known sites worldwide. Most sites experienced episodic hypoxia, which is a precursor to development of seasonal hypoxia. The Baltic Sea coastal zone displays an alarming trend with hypoxia steadily increasing with time since the 1950s effecting nutrient biogeochemical processes, ecosystem services, and coastal habitat.

The Development of the Baltic Sea Basin During the Last 130 ka

During the Eemian interglacial 130–115 ka BP, the hydrology of the Baltic Sea was significantly different from the Holocene. A pathway between the Baltic basin and the Barents Sea through Karelia existed during the first ca. 2.5 ka of the interglacial. Both sea surface temperature and salinity of the SW Eemian Baltic Sea were much higher, ca. 6°C and 15‰, respectively, than at present. A first early Weichselian Scandinavian ice advance is recorded in NW Finland during marine isotope stage (MIS) 4 and the first Baltic ice lobe advance into SE Denmark is dated to 55–50 ka BP. From the last glacial maximum that was reached ca. 22 ka BP, the ice sheet retreated northward with a few still-stands and readvances; however, by ca. 10 ka BP the entire basin was deglaciated. Weak inflows of saline water were registered in the southern and central Baltic Sea ca. 9.8 ka BP with full brackish marine conditions reached at ca. 8 ka BP and the maximum Holocene salinity was recorded between 6 and 4 ka BP. The present Baltic Sea is characterized by a marked halocline preventing the vertical water exchange resulting in hypoxic bottom conditions in the deeper part of the basin.

Calendar year ages of three mid-Holocene tephra layers identified in varved lake sediments in west central Sweden

Three intermediate to rhyolitic Icelandic Holocene tephra horizons (Hekla-3, Kebister, and Hekla-4) have been identified in annually laminated (varved) lake sediments in the Province of Varmland, west central Sweden. Calendar year ages were obtained from varve counts and are supported by additional C-14 dating based on terrestrial plant macrofossils. The varve ages of the three tephras are 3295 +/- 95 cal. BP (Hekla-3), 4030 +/- 103 cal. BP (Kebister), and 4390 +/- 107 cal. BP (Hekla-4). The varve age of Hekla-3 refines the former calibrated 14 C age based on studies in the British Isles and Germany. Considering the age-error estimates associated with the varve chronology and calibrated C-14 ages, the ages of Kebister and Hekla-4 are in agreement with former studies. Thus, the age difference between Kebister and Hekla-4 is estimated to be ca 400 +/- 40 varve years (formally estimated to ca 200 C-14 years), between Hekla-3 and Hekla-4 to ca 1135 +/- 55 varve years (formally estimated to ca 1100 calibrated C-14 years) and between Hekla-3 and Kebister, 708 +/- 20 varve years. (Less)

Occurrence of varved lake sediment sequences in Varmland, west central Sweden: lake characteristics, varve chronology and AMS radiocarbon dating

Varved lake sediments can be used to set multiple environmental proxies within a calendar year time scale. We undertook a systematic survey of lakes in the Province of Varmland, west central Sweden, with the aim of finding continuous varved lake sediment sequences covering the majority of the Holocene. In Fennoscandia, such sediments have previously only been recorded in northern Sweden and in southern and central Finland. By following a selective process and fieldwork we discovered three new varved sites (i.e. Furskogstjarnet, Motterudstjarnet and Kalksjon). We found that lakes with varved sediments have several common lake morphometry properties and lake catchment characteristics such as maximum water depth, maximum water depth/lake surface area ratio, catchment soil types, altitude and number of inflows. Varve chronologies, supported by AMS- C-14 dating and tephrochronology were established for two of the sediment profiles. These varve chronologies are the longest geological records with an annual resolution known to exist in Sweden. In Furskogstjarnet, the AMS- C-14 dates based on terrestrial plant macrofossils at several levels deviate significantly from the varve based time-depth curve. In Motterudstjarnet, a fully reasonable time-depth model based on the C-14 dates gives older ages in the lower part of the sequence compared to the varve chronology. These results highlight that seemingly acceptable AMS radiocarbon dates may be erroneous. They also point to the fact that varved lake sediments are reliable geological archives with respect to chronological control and accuracy. Thus, these archives should be of prime interest for studies of climate and environmental change undertaken with the aim of providing sub-decadal resolution proxy data sets. (Less)

Bulk sediment 14C dating in an estuarine environment – How accurate can it be?

Abstract Due to a lack of marine macrofossils in many sediment cores from the estuarine Baltic Sea, researchers are often forced to carry out 14C determinations on bulk sediment samples. However, ambiguity surrounding the carbon source pathways that contribute to bulk sediment formation introduces a large uncertainty into 14C geochronologies based on such samples, and such uncertainty may not have been fully considered in previous Baltic Sea studies. We quantify this uncertainty by analyzing bulk sediment 14C determinations carried out on densely spaced intervals in independently dated late-Holocene sediment sequences from two central Baltic Sea cores. Our results show a difference of ~600?14C?yr in median bulk sediment reservoir age, or R(t)bulk, between the two core locations (~1200?14C?yr for one core, ~620?14C?yr for the other), indicating large spatial variation. Furthermore, we also find large downcore (i.e., temporal) R(t)bulk variation of at least ~200?14C?yr for both cores. We also find a difference of 585?14C?yr between two samples taken from the same core depth. We propose that studies using bulk sediment 14C dating in large brackish water bodies should take such spatiotemporal variation in R(t)bulk into account when assessing uncertainties, thus leading to a larger, but more accurate, calibrated age range. (Less)