Argument list is changing whenever I execute a change to a separate list with removerange

Question

I'm passing a list as argument, and creating a second one inside a method, but when I change the second one, the changes are also being applied on the first. Am I missing something?

Appreciate any hint on this

    //Creating List method
    public List<(string pro_description, DateTime pro_opentimes)>  Analysis_simple_FW_ListFW_npos_mposapos(List<(DateTime pro_opentimes, double pro_opencost)> arg_list0, int arg_npos, int arg_mposapos)
    {

            //The return list
            List<(string pro_description, DateTime pro_opentimes)> list1
                = new List<(string pro_description, double pro_opencost)>();

            //A list to store the arg_list0 that is used as an argument
            List<(DateTime pro_opentimes, double pro_opencost)> arg_list1 = arg_list0;


            int n2 = arg_list0.Count;
            if (n2 > 0)
            {

                if (arg_list1.Count() >  arg_npos * arg_mposapos)
                {
                    //I change here the arg_list1.. hoping only arg_list1 is changed
                
                    arg_list1.RemoveRange(0, (arg_list1.Count() / arg_mposapos - arg_npos *  arg_mposapos) );
               


                }

            

              if (par_checklive_AnalysisListBar)
                {

                  //I realize here, that arg_list0, in the end, also has entries removed.. Why??
                  Print("CHECKLIVE: Analysis_simple_FW_ListFW_npos_mposapos(  arg_list0.Count:" + arg_list0.Count + " arg_list1.Count:" + arg_list1.Count); 

                }

                list1.Add(("Start: ", arg_list1[0].pro_opencost, dtUTCtoLocal(arg_list1[0].pro_opentimes), 0, 0, (0, 0, 0)));
            }

                return list1;
                }