## 1 Introduction  
Global warming has attracted widespread attention around the world, and more and more countries are devoting their efforts to carbon emission reduction actions [1, 2, 3, 4]. As a low-carbon and environmentally friendly green mode of transport, bikeshare is receiving increasing attention in the process of sustainable urbanization, with its features of reducing vehicle miles traveled (VMT), lowering vehicle emissions, easing traffic congestion, increasing commuting alternatives, facilitating public transit use, high travel convenience, and low use costs [5, 6, 7, 8].  
In many developing countries, over-reliance on car travel has become the main cause of urban environmental pollution, traffic congestion, and social equity [9], and prioritizing public transit (buses, metros, etc.) provides the answer to these problems [10, 11]. Bikeshare as a feeder mode of metro stations (BS-FMMS) can effectively alleviate the first- and last-mile travel problems. The integration of bikeshare and metro facilitates multimodal travel in urban areas, making it a hot topic in urban transportation research. Relevant studies focus on transfer characteristics [12, 13], use frequency [14, 15], route analysis [16, 17], accessibility [18, 19], and the impact of the built environment [20, 21].  
Many existing studies have hypothesized that bikeshare travel may replace other modes of transport (i.e., those that have an adverse impact on the environment), leading to reduced environmental impacts [22; 23; 24]. Carbon emission reduction has been a popular research topic for the environmental benefits of bikeshare [25; 26]. The integration of bikeshare and metro is a special type of bikeshare cycling behavior [20], which also has the potential to be a substitute for environmentally destructive modes of transport, resulting in the carbon emission reduction benefits of bikeshare. However, two research gaps remain in the existing studies: (1) How does one measure the carbon reduction benefits of BS-FMMS? (2) What are the spatiotemporal differences in the carbon reduction benefits of the BS-FMMS between workdays and non-workdays?  
To fill the above research gaps, this paper uses bikeshare data and metro station data in Shenzhen, China, to study the carbon emission reduction benefits of BS-FMMS and its spatiotemporal differences between workdays and non-workdays. First, a framework for measuring the carbon emission reduction benefits of BS-FMMS is established for identifying the transfer behaviors of bikeshare and then measuring the carbon emission reduction benefits of bikeshare. Next, the carbon emission reduction of BS-FMMS is counted into the corresponding time pane and spatial unit, and the spatiotemporal differences between workday and non-workday are analyzed by using spatiotemporal visualization techniques. The study findings can serve as valuable references for the relevant departments involved in developing carbon reduction plans for urban transportation.  
The rest of the paper is organized as follows: Section 2 reviews the related work; Section 3 introduces the study area and data; Section 4 presents the analysis framework and specific methods of carbon emission reduction benefits of BS-FMMS; Section 5 discusses the carbon reduction benefits of BS-FMMS by combining the results of spatiotemporal visualization; and Section 6 summarizes the results and provides a research outlook.