Diatom Stratigraphy and Relative Sea Level Changes of the Eastern Baltic Sea Over the Holocene

Ränivetikate stratigraafia ja Läänemere idaosa veetaseme muutused Holotseenis Väitekiri käsitleb Läänemere idaosa pärastjääaegse rannajoone siirdeid, glatsioisostaatilise maakerke iseärasusi ja rannikumere soolsuse muutuseid. Uurimispiirkond hõlmab Eesti põhja- ja loodeosa, Hiiumaa ning Riia lahe lõunarannikut. Biostratigraafiliste meetoditega (ränivetikad, õietolm) ning litostratigraafiliste parameetritega (setete kuumutuskadu, magnetiline vastuvõtlikkus, lõimis) määrati eri kõrgusel asetsevate settebasseinide läbilõigete isolatsiooni tasemed. AMS radiosüsiniku dateeringute põhjal määrati uuritavate settebasseinide isoleerumise aeg tagades nii rannajoone rekonstruktsioonidele täpse ajaskaala. Tuginedes GIS ja DMT mudelitele rekonstrueeriti Läänemere erinevate staadiumite rannajooned ning kolmemõõtmelised paleogeograafilised kaardid. Läti ala kohta on tegemist esmakordse sellise detailsusega uuringuga.

Uuringuala arvutuslik suhteline glatsioisostaatilise maakerge on suurim Hiiumaal, Litoriinamere transgressiooni ajal tõusis maapind Kõpus 3,4 mm aastas ja Prassis 2,8 mm aastas, tänapäeval on maakerge tunduvalt väiksem, vastavalt Kõpus 2,5 mm aastas ja Prassis 2,1 mm aastas. Sarnane suhtelise maakerke kiiruse vähenemine on omane ka Eesti loode- ja põhjaosale. Kui Litoriinamere transgressiooni ajal tõusis maapind Tänavjärve ümbruses 2,8 mm aastas ja Lohjal 2,4 mm aastas, siis tänapäevaks on maakerge kahanenud Tänavjärve ümbruses 2,2 mm aastas ja Lohjal 2,0 mm aastas. Riia lahe lõunarannikul on maapind vajunud. Ajavahemikul 6500 ja 4600 aastat tagasi on arvutuslik suhteline maa vajumiskiirus 0,8 mm aastas, kuni tänapäevani on maapinna alanemise kiirus püsinud 0,1 mm aastas.

Rannasiirdekõverad kinnitavad soolaseveelise Litoriinamere transgressiooni maksimumi eriaegsust. Hiiumaal Kõpu poolsaarel Kõivasoo lähistel oli Litoriinamere veetase maksimaalne 8800 aastat tagasi, seevastu Lõuna-Hiiumaal Prassis 8200 aastat tagasi. Litoriinamere transgressiooni maksimum Põhja-Eestis oli veelgi noorem, 7800 aastat tagasi. Litoriinamere transgressiooni maksimumi eriaegsust Läänemere idaosas põhjustas erinev maakerke kiirus. Litoriinamere transgressiooni maksimum Riia lahe lõunarannikul on veelgi noorem ja pikemaajalisem protsess, mis kestis ajavahemikul 6700-4200 aastat tagasi. Erinevalt Läänemere lääneosast, kus Litoriinamere arengus esines mitu transgressiooni, esines Läänemere idaosas vaid üks meretaseme tõus. Kuna Eesti uurimisalal glatsioisostaatiline maakerge ületas maailmamere eustaatilist tõusu, siis järgnes Eestis Litoriinamere aegsele transgressioonile meretaseme ühtlane langus. Seevastu Läti alal rannasiirdekõver järgib globaalset ookeani veetaseme eustaatilist tõusukõverat.

Abstract

The main objectives of this thesis are to reconstruct changes of relative sea level (RSL) and to determine the pattern of land uplift in the eastern Baltic over the Holocene. The reconstruction is based on isolation basin studies determined after the isolation contact in the isolation basins, the height of the threshold, and geomorphological markers. Other issues discussed in this thesis include the number of Litorina Sea (LitS) transgressions in the eastern Baltic Sea, credibility of the data, and the number of study sites in areas with complicated sediment stratigraphy.

Lake sediment sequences from five lakes in northern and northwestern Estonia, three from the Island of Hiiumaa and eight from the southern coast of the Gulf of Riga were analysed by using multi-proxy studies such as diatom analysis, loss-on-ignition (LOI), magnetic susceptibility (MS), grain size analysis, Accelerator Mass Spectrometry (AMS) and conventional 14C datings. Additionally, spheroidal fly ash particles were used to date upper loose sediments of Lake Lilaste. Finally, palaeogeographical reconstructions were based on GIS analysis in which interpolated water-level surfaces were removed from the digital terrain model (DTM).

The obtained data were applied for reconstructions of RSL curves that enabled the detection of water-level fluctuations and patterns of land uplift in northern and northwestern Estonia, the Island of Hiiumaa, and the southern coast of the Gulf of Riga. Water-level curves from northern Estonia and the Island of Hiiumaa show a smooth decrease in RSL and land uplift, which have been close to linear
since the mid-Holocene. Data from the surrounding areas of Riga confirm that the LitS transgressed consistently at the southern coast of Gulf of Riga, representing a pattern of eustatic sea level (ESL) changes.

The reconstructed water-level curves display a diachronous LitS transgression peak in the Island of Hiiumaa that occurred earlier in the Kõivasoo area at about 8800 cal BP and later in the Prassi area at approximately 8200 cal BP (Paper IV). The LitS maximum in northern Estonia at about 7800 cal BP (Paper II) was later than that at the Island of Hiiumaa. The time-transgressive LitS transgression was
observed in the Baltic Sea region as a result of the different rates of land uplift. The LitS transgression culminated almost 1000 years later and was a long-lasting event (about 2500 years) in the southern coast of the Gulf of Riga (Paper V) compared with sites at high isolines in the northern part of the Baltic Sea.

The calculated apparent land uplift during the LitS transgression in the Island of Hiiumaa was 3.4 mm yr-1 at Kõivasoo and 2.8 mm yr-1 at Prassi and has decreased to 2.5 and 2.1 mm yr-1 at present (Paper IV). Similar declines of land uplift are observed in northern and northwestern Estonia. The uplift rate during LitS transgression was approximately 2.8 mm yr-1 in the area of Tänavjärv, which has decreased to 2.2 mm yr-1at present, and at Lohja from 2.4 mm yr-1 to about 2.0 mm yr-1 (Paper II). However, preliminary calculations of the apparent uplift at Lilaste in the southern coast of Gulf of Riga reveals that this area experienced minor subsidence of about -0.8 mm yr-1 between 6500 cal BP and 4600 cal BP,and during the last 4600 years, the apparent land uplift rate has been near -0.1 mm yr-1.

Lilaste and other lakes in the surrounding areas of Riga indicate a complicated sediment deposition and changes in RSL over the Holocene in the area with the apparent land uplift close to 0 mm yr-1. The sediment sequences of the studied lakes contain evidence of low water-levels, such as peat/soil layers and buried wood, during the Yoldia Sea (YS), Ancylus Lake (AL), and Early LitS stages, as
well as slow water-level increases, river bifurcation, and intermittent LitS brackish-water inflows that promoted nutrient enrichment in the isolation basins (Paper V). In only two of the eight studied basins Lilaste and Slokas, the diatom analysis confirmed that the LitS reached the threshold and influenced their environment. According to the pattern of the reconstructed water-level curves, the RSL of the Baltic Sea in the early Holocene was related primarily to deglaciation dynamics, although since about 8500 cal BP, the key role has been the interplay between glacio–isostasy and ESL rise.

Similarities in diatom assemblages were observed in the studied isolation basins, such as the mass occurrence of small-sized Fragilaria spp. and increased nutrient content. The mass occurrence of small-sized fragilarioid taxa just before, during, or after the isolation has been recognised by several authors in different studies. Fragilaria spp. indicates a period of environmental instability due to increased turbidity and higher nutrient content (Paper I–V). The peak of Stephanodiscus parvus and Cyclotella meneghiniana, indicating an increased nutrient content and a high conductivity state, are identified in the basins in which isolation from the sea was a long-lasting event (Paper I–III, V). The enhanced nutrient content might be explained by occasional mixing of brackish water and fresh water, which promotes biological productivity.

This thesis demonstrates the value of the multi-proxy approach in a wide number of settings, particularly the use of diatom analysis, to determine the palaeoenvironmental changes and to reconstruct the RSL. The obtained data and conclusions contribute to earlier knowledge of water-level change and shoreline displacement in the eastern Baltics. The approach used to study the southern coast of the Gulf of Riga, including well-dated Lake Lilaste, can be used as the basis for further investigation in other areas along the Latvian coastline.