LOWELL — Transparency, trust and communication — and almost 168,000 gallons of diesel fuel — were the themes of Tuesday night’s joint meeting of the City Council Environmental & Flood Issues and Neighborhoods subcommittees. Markley’s most recent request joins previously approved petitions for 68,000 gallons of fuel. If approved by the council, almost 168,000 gallons of diesel fuel for emergency backup generators would be stored on site. Neighborhood density and the impact of Markley’s operations on the residents was on Councilor Kim Scott’s mind. Councilor Rita Mercier, speaking as a resident, suggested that approval of Markley’s petition be postponed until neighborhood conditions are met.