The full acknowledgement of your sources is essential, of course. You are not expected to originate much data as an undergraduate: your unique contribution stems from your ability to synthesise and critique the work of others. Consequently, pretty much all of the material you introduce requires a citation. As a rule, I would say it is better to over-cite, rather than under-cite (unless you are drawing repeatedly from the same source). Very obvious points don’t require a citation, but don’t assume that the material you are introducing is common knowledge: for example, most scientists accept the reality of anthropogenic global warming, but it might still be best to provide some quantitative data and cite the IPCC.
The Harvard system (author, date) is preferred for citations and references: it is simple to apply and allows the marker to quickly see the range and quality of your sources. Note that there are many ways to format the reference list (as opposed to bibliography) at the end of your essay (each scientific journal uses a slightly different format), so find something that a) works for you and b) includes all the relevant information. It is particularly important to learn how to cite and reference websites. Don’t bother including sources in your reference list that you have not cited.
Don’t cite lectures:
After the dissipation of the earth’s primary atmosphere, volcanic activity was one of the major contributors to the secondary atmosphere and various climatic and chemical alterations thereafter (Donovan, A. [lecture] 9/10/11).
The lecturer should have used a reliable published source (possibly their own research): find out what this source is and use that. Citing lectures is just lazy.
The Harvard system simply requires an author’s surname and the date of the publication. For example:
Pinatubo’s radiative forcing exceeded for 2 years the warming effects of anthropogenic greenhouse gases (P Francis, Volcanoes)
Should have been:
Pinatubo’s radiative forcing exceeded for 2 years the warming effects of anthropogenic greenhouse gases (Francis 1993)
The Harvard system allows sources to be cited in a very succinct fashion. For example:
Oppenheimer’s description of The Late Pleistocene Eruption of Young Toba Tuff pulls together details of how ash was ejected from the volcano
Could simply be:
Oppenheimer (2011) pulls together details of how ash was ejected from the volcano
Very specific statements, especially where they contain quantitative data, absolutely require citations. For example:
After the 1982 El Chichon eruption the globally averaged stratospheric temperature rose by about 18C for about 2 years.
Has to be substantiated with reference to published sources.
Certain statements also require citations. For example:
[aerosols] absorb Earth’s thermal radiation, warming the atmosphere they occupy. However this process is negligible when coupled with the cooling effect of the aerosol clouds, and so producing a net drop in temperature.
Who says it is negligible? What is the evidence for this? A citation is definitely required.