This technical note presents the new automated in situ cosmogenic carbon-14 extraction system at PRIME lab. Improvements and advantages of the new installations compared to the previous system are described. Tests with different sample boat materials and various Lithium Metaborate flux batches show the impact of these materials on the procedural blank levels. Experiments with different preheating and extraction temperatures demonstrate the importance of these temperatures for the 14C recovery from the largely used inter-comparison material CRONUS-A and are therefore very useful for other in situ 14C labs.

The manuscript reads very well and the data is very well presented and described. Below I only list a few technical suggestions that could be considered.

The title is a bit bulky and could be clarified in my opinion. In particular I find a few terms confusing: First, it only becomes clear in the text that you use the term « combustion » to refer to the preheating step that removes atmospheric/organic C. Isn’t heating to 1000-1100°C during the in situ extraction also combustion? Secondly, people who are familiar with in situ 14C extraction procedures would associate “high-temperature extraction” with systems that do not use the LiBO2 flux (heating at much higher temperatures instead). Maybe rather change the title to “Studying Li-metaborate fluxes and preheating and extraction temperatures with a new, fully automated in situ cosmogenic 14C processing system at PRIME Lab”?

Lines 140-141: Please specify whether or not you also add He on Day 2 when you insert the sample.

Lines 241-251: Concerning the high 14C and C blanks from the various LiBO2 batches: Isn’t the heating on Day 1 supposed to degas all the CO2 in the LiBO2? Was a longer degassing duration tested to reduce the blanks? BTW, the heating temperature on Day 1 is not specified. Is it always the same the extraction temperature on Day 2?

Lines 290-296: It would have been interesting to test the 500°C preheating with the Al2O3 boats. Can you exclude that the 10% difference in 14C is due to absorption by the Al2O3?

Lines 303-315: For the CRONUS-A extractions in the Pt/Rh boat, there seems to be slight trend though of 14C concentrations increasing with extraction temperature: 4 of the 5 extractions at 1100°C are systematically higher than those at 1050° (3h) and 1000° (4.5h). It would be interesting to test further if there is systematic impact or not. Especially because the mean of CRONUS-A concentrations at ETH being the highest of all published values (ref your Fig. 6), was derived with a higher extraction temperature than elsewhere.

Fig. 6: It would be helpful if the 1sigma band of the PCEGS mean was shown, and also to clarify on the figure if PCEGS-47 and -50 included in that mean value.

Table 4: Shouldn’t the first three samples get a footnote saying that they were combusted at 600°C?

Irene Schimmelpfennig

In this technical note, the authors describe the new automated in situ cosmogenic carbon-14 (C-14) extraction system at Purdue University/PRIME Lab. The system is based on previous designs by N. Lifton and B. Goehring (formerly at Tulane) which use LiBO₂ flux to dissolve quartz sample grains, but upgraded with a glass coil to trap gases during sample extraction, and two extraction ovens isolated from the rest of the extraction/purification/graphitization processes, allowing for overlap. The paper presents the results of blank and interlaboratory standards (CRONUS-A, CoQtz-N) using different bottles of pre-fused LiBO₂ flux beads, various combustion and fusion temperatures/timings, and Al₂O₃ vs. Pt/Rh sample boats. The authors show that: (1) Pt/Rh boats heat more quickly and provide much lower blanks than Al₂O₃ boats, (2) a too aggressive combustion step results in diffusive loss of C-14, (3) bottles of flux from a certain manufacturer result in higher and increasing blanks, (4) standard measurements on the Purdue system are in line with accepted values from other laboratories.

For the most part the data contained in this technical note is presented well. I offer here some items that could be clarified for those less familiar with in situ C-14 extraction, and other suggestions/questions:

In general, I think a flow diagram of the procedures with approximate timings would be helpful. I found the discussion of “Day 1/Day 2 procedures” and degassing vs. combustion vs. extraction difficult to follow in the text.

Lines 81-82. It would be nice to briefly describe the new CEGS LN₂ distribution system since the issues with the former design were discussed in detail.

Line 105: A comma after “downtube” would make this sentence clearer.

Lines 152-154: Specify which steps/traps are removing which gases.

Lines 197-198: Specify units of Bg (i.e., blank value in C-14 atoms or as ¹⁴C/C_total), and perhaps add a brief sentence on why this relationship exists.

Line 221: Does the etching pattern necessarily mean that the Pt/Rh boat heats more evenly, or just faster? The etching patten of the Al₂O₃ boats in Fig. 3 shows etching of the tube along the full length of the boat.

Line 257: These two blanks are listed as Claisse Pure (“Batch 4”) on Fig. 4, but described as Ultra-Pure in the text.

Lines 284-288: Add a short sentence or two describing the two quartz standards used (CRONUS-A, CoQtz-N). E.g., where they come from, approximate age/exposure history, etc.

Fig. 3: Either use “(A)” or “A)” for consistency.

Fig. 4: Can the LiBO₂ Batch #s listed on the Figure (Batches 1-4) be updated to reflect the Batch #s/IDs used in the text (or can you list the Batch # used in Fig. 4 along with the actual bottle batch # in the text?) so the reader can more easily follow which blanks are from which bottle?

Fig. 6: Can you add shading around the PCEGS mean value representing the 1 std dev (similar to how the Jull et al., 2015 data is presented)?

Tables 2, 3: Can you note in the tables which blanks used which batch/bottle of flux beads?