Holocenski sedimenti Krnskega jezera

raziskovalna naloga

Urška Anžlovar (Avtor), Andrej Šmuc (Mentor), Andrej Novak (Komentor)

Povzetek

V sklopu raziskovalne naloge smo preučili dve jedri sedimentov Krnskega jezera, globoki 73 cm in 92 cm. Za obe jedri smo izdelali detajlni sedimentološki profil ter odvzeli vzorce za granulometrijsko analizo, za eno jedro pa smo odvzeli tudi vzorce za datiranje s pomočjo radionuklidov 210Pb in 226Ra. Iz sedimentološkega profila lahko ugotovimo, da sta obe jedri sestavljeni iz tankoplastovitih in laminiranih sedimentov. Plasti in lamine so sestavljene iz gline, glinastega melja, melja, peščenega melja ter peska v različnih odtenkih sive, ponekod nekoliko zelenkaste ter rjavkaste barve. Sediment je v nekaterih laminah in plasteh normalno gradiran. Prav tako v obeh jedrih najdemo veliko ostankov organskega materiala ter redkeje tudi posamezne večje klaste. Rezultati granulometrijske analize so pokazali, da v obeh jedrih močno prevladuje melj, medtem ko je v prvem jedru prisotnega nekoliko več peska, v drugem pa nekoliko več gline. V jedru KRN01 20 od 50 vzorcev predstavlja glinasto meljast pesek, ostalih 30 pa glinasto peščen melj. Največ peska se pojavi na globini 11 cm, in sicer 88% vzorca, melja na globini 67,5 cm s 83 % ter gline na globini 43 cm s 7,4% vzorca. V jedru KRN09 sedem vzorcev predstavlja glinasto meljast pesek, en vzorec meljasta glina, ostalih 62 pa glinasto peščen melj. Največ peska se pojavi na globini 2,5 cm in sicer 66% vzorca, melja na globini 57,5 cm s 86,9,% ter gline na globini 44 cm s 76,5% vzorca. Za jedro KRN09W smo izvedli datacijo vzorcev sedimenta s pomočjo radionuklidov 210Pb in 226Ra. Rezultati so pokazali, da je bila hitrost sedimentacije 0,072 cm ± 0,005 cm na leto, kar pomeni, da je jedro do globine 93 cm staro ~ 1292 let. Na podlagi detajlno posnetega profila jeder, strukture sedimenta, sedimentnih tekstur ter interpretacijskih diagramov CM (Passega 1964) sklepam, da je večino sedimentov nastalo z gravitacijskimi (najverjetneje turbiditnimi) tokovi. Le manjši del sedimenta (predvsem v jedru KRN09) se je usedal s prevladujočo pelagično sedimentacijo. Za sedimente Krnskega jezera je značilna tudi visoka vsebnost makroskopsko opazne organske snovi (organski drobir). Na podlagi makroskopskih opazovanj, ugotavljam, da gre večinoma za ostanke višjih kopenskih rastlin. Ta organski material je bil skupaj s sedimentom spran v jezero iz obal jezera in iz pojezerja. Glede na določeno povprečno starost jezerskih sedimentov bi se lahko grobozrnati sediment na globini 0-1 cm usedel kot posledica potresov (leti 1998 in 2004) ter tudi kot posledica velike količine padavin, kot na primer novembra 2000. Sediment na globini 4.5-5.5 pa lahko delno pripisujemo potresom leta 1942 in 1944. Vidna je tudi izrazita razlika v sedimentaciji v različnih delih jezera. Dno jezera je namreč sestavljeno iz dveh depresij, ki sta med seboj ločeni z grebenom. V depresiji, ki je proksimalna delti (jedro KRN01), se sedimentira bolj grobozrnata frakcija kot v distalni depresiji na severovzhodni strani jezera (jedro KRN09). Ti različni sedimentacijski pogoji tudi onemogočajo neposredno lateralno korelacijo obeh odvzetih vrtin.

Ključne besede

Krnsko jezero;jezerski sedimenti;granulometrija;datacije;transport;holocen;

Podatki

Jezik:	Slovenski jezik
Leto izida:	2020
Tipologija:	2.11 - Diplomsko delo
Organizacija:	UL NTF - Naravoslovnotehniška fakulteta
Založnik:	[U. Anžlovar]
UDK:	55
COBISS:	31744771
Št. ogledov:	454
Št. prenosov:	134
Ocena:	0 (0 glasov)
Metapodatki:

Ostali podatki

Sekundarni jezik:	Angleški jezik
Sekundarni naslov:	Holocene sediments of lake Krn
Sekundarni povzetek:	As a part of the research work, two cores of Krn Lake sediments, 73 and 92 cm deep, were analysed. Detailed sedimentological profiles were made for both cores and samples for granulometric analysis were taken. For one core, samples for dating were taken by using radionuclides 210Pb and 226Ra. Sedimentological profiles show that both cores consist of thin-layered and laminated sediments. They are composed of clay, clayey silt, silt, sandy silt and sand of gray, sometimes slightly greenish and brownish colour. Sediment is in some layers and in laminae normally graded. Inside some of the laminae in both cores, remains of organic material and individual dropstonescan be found. The results of granulometric analysis showed that the silt strongly dominates in both cores, with slightly more sand in the first and slightly more clay in the second core. In the core KRN01, 20 of the 50 samples represent clayey silty sand, and the other 30 clayey sandy silt. Most sand occurs at a depth of 11 cm, 88% of the sample, silt at a depth of 67.5 cm with 83% and clay at a depth of 43 cm with 7.4% of the sample. In the core KRN09, seven samples represent clayey silty sand, one sample silty clay, and other 62 samples clayey sandy silt. Most sand occurs at a depth of 2.5 cm, 66% of the sample, silt at a depth of 57.5 cm with 86.9% and clay at a depth of 44 cm with 76.5% of the sample. For the core KRN09W, sediment dating was performed by using radionuclides 210Pb and 226Ra. The results showed that sedimentation rate was 0,072 cm± 0,005 cm per year, meaning that the core at the depth of 93 cm is ~ 1292 years old. Based on a detailed profile of the cores, sediment structure, sediment textures, and CM interpretation diagrams (Passega 1964), I conclude that most sediments were formed by gravitational (most likely turbidite) flows. Only a small portion of the sediment (mainly in the KRN09 core) was deposited with predominant pelagic sedimentation. The sediments of Lake Krn are also characterized by a high content of macroscopically noticeable organic matter (organic debris). Based on the macroscopic observations, I find that these are mostly remnants of higher terrestrial plants. This organic material, along with a sediment, was washed into the lake from the shores of the lake and from the lake district. Based on a determined average age of lake sediments, coarse-grained sediment could settle at a depth of 0-1 cm as a result of the earthquakes, which happened in 1998 and 2004, and as a result of high rainfall, such as the one that occurred in November 2000. Sediment at a depth of 4.5-5.5 could be partly attributed to the earthquakes of 1942 and 1944. There is also a marked difference in sedimentation in different parts of the lake. The bottom of the lake consists of two depressions, which are separated by a ridge. In the depression that is proximal to the delta (core KRN01), a coarser-grained fraction is sedimented than in the distal depression on the northeast side of the lake (core KRN09). These different sedimentation conditions also preclude a direct lateral correlation of the two cores.
Sekundarne ključne besede:	lake Krn;lake sediments;granulometry;dating;transport;Holocene;
Vrsta dela (COBISS):	Zaključna naloga
Študijski program:	0
Konec prepovedi (OpenAIRE):	1970-01-01
Komentar na gradivo:	Univ. v Ljubljani, Naravoslovnotehniška fak., Odd. za geologijo
Strani:	XIII, 60 str.
ID:	12046531