Chemistry and early diatom studies

Cleve's early research, in 1895 and 1896, included studies of diatoms in the high-altitude lakes in the Lule Lappmark region. She published work identifying and drawing newly discovered diatoms from Arctic lakes. She also surveyed the plant ecosystems in the far north regions and their adaptations to the harsh environment.^[5] Between 1896 and 1898, Cleve published four chemistry papers, all of which concerned nitrogenous organic chemicals in varying structures. Her research on ytterbium, also performed at Uppsala University, was published after she moved to Stockholm University; she discovered the atomic weight and various other properties of the element.^[7] She obtained her doctoral degree in May 1898 at Uppsala University, 23 years old,^[5] on a thesis entitled Studier ofver några svenska väksters groningstid och förstärkningstadium,^[10] "Studies on the germinating time and the juvenile stage of some Swedish plants". She was the second Swedish woman to do so, and the first in a scientific discipline. From 1898 to 1902, she was employed as an assistant professor of chemistry at the Chemical Institution at Stockholms högskola (later Stockholm University) ^[7] which proved progressive in its willingness to hire women. During her tenure at Stockholm, she published a paper on lanthanum and selenium.^[6][11] She left the chemistry department upon her marriage to Hans von Euler-Chelpin, with whom she published 16 papers in the five years after her departure. The couple worked on nitrogenous organic compounds, the synthesis of ketoses from formaldehyde, metal-ammonia complexes, the chemicals in resin, and industrial alcohol synthesis. While a teacher at various Stockholm secondary schools, Cleve resumed her research on plankton, publishing major studies on the flora in bodies of water near Stockholm in 1910 and 1912. These papers are still of significance today as they are the only record of diatomaceous plankton before pollution occurred in the Stockholm area. While still a secondary school teacher, in 1913, Cleve was hired as a biological assistant by the Swedish Hydrographical Biological Commission. For them, she produced a 1917 monograph on plankton research in the Skagerrak strait. After her move to a position in the forestry lab of the Uddeholm Company, Cleve continued to research. Her research there, undertaken between 1920 and 1925, comprised 23 papers on various topics, including the lignin produced during sulfite pulp manufacturing and how to determine the lignin content of a particular wood, the composition of pine and spruce needles, carbon dioxide's role in plants, methods for separating pulp by-products, petroleum, and coal.^[7] This work was mainly focused on lignin chemistry, however.^[6] Her connection to teaching continued as, during this period, she also authored a popular science book on selenium as well as an introductory textbook in applied biochemistry.^[7]

Quaternary geology and later diatoms studies

Around this time, in the later 1920s and through the 1960s, Cleve's research refocused again on both living and fossil diatoms in the Baltic Sea. Her research also extended into related paleobotanical issues, including the changes in water level of the Baltic Sea, then an inland sea, in the late Ice Age and the period shortly after.^[7] Cleve performed boundary analyses derived from diatom studies to determine the changes in the Baltic's connection with the ocean; these are considered to have dubious validity because of the possibility of redeposited diatoms in the sediment.^[8] In her discourse with the contemporary Scandinavian scientific establishment, Cleve found conflict as a proponent of the oscillation theory.^[8] This theory was first proposed by N.O. Holst in 1899 and was then recycled by Ernst Antevs in 1921. When Cleve refashioned the theory in a 1923 publication it was again rejected by established geologists. The theory held that Fennoscandia's surface had oscillated up and down like a pendulum losing momentum after the Fennoscandian Ice Sheet melted. Her insistence on the validity of the theory at the Geological Society of Stockholm extended about a year until she was expelled from it.^[12] In 1927 and 1928 she was involved in controversy through opinion pieces in newspapers with geologist Henrik Munthe. Munthe had proposed to declare "Svea River" at Degerfors a national monument. Cleve argued that "Svea River" should be made a national monument but that it was not the outlet of the ancient Ancylus Lake as Munthe and von Post claimed.^[12] The controversy turned personal when as Munthe defended his geological interpretation in the newspapers she responded by accusing him of having unscientific reasons to advance his idea of what Svea River was.^[12]

During the 1932–1955 period she published several monographs on diatom taxonomy. The first, in 1932, covered 535 extant and fossil diatom species—including 184 species unknown in Sweden—found in the Täkern Basin. The second followed two years later, after 1932 fieldwork in Lapland; it covered 673 northern Finnish diatom species, many of which were new discoveries. This work led her to geological studies undertaken through examination of diatom flora. That same year, Cleve published on the Quaternary geology of the region. In this same period, she was employed by geologists of the Geological Survey of Sweden to study diatoms in postglacial sediment. She did not produce another major work until 1951, when her comprehensive monograph on Swedish and Finnish diatoms, written over more than a decade, was published. Still in use today, Die Diatomeen von Schweden und Finnland covered approximately 1600 diatom species and their taxonomy, distribution, ecology, and fossils. She continued work on these regions' flora for many years, discovering new species and correcting taxonomy.^[8] After her 1945 return to Uppsala University, Cleve participated in the geology department and contributed theories about changes in the Baltic Sea's water level during the Quaternary period. She lectured on diatomology there from 1947 to 1948 at the University's Institute of Plant Ecology. In 1948, she was awarded an honorary degree, as Sweden's first female recipient of the Jubilee Doctor of Philosophy. She was awarded an honorary professorship for her diatom studies in 1955.^[9] Cleve continued to publish scientific papers until the age of 86.^[6]